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  1. phosphorus and phosphorus control
    by "Roger S. Miller" <rgrmill/> (Wed, 17 Nov 1999)
  2. phosphorus and phosphorus control (2)
    by "Roger S. Miller" <rgrmill/> (Thu, 18 Nov 1999)
  3. phosphorus and phosphorus control (3)
    by "Roger S. Miller" <rgrmill/> (Sat, 20 Nov 1999)
  4. phosphorus and phosphorus control (4)
    by "Roger S. Miller" <rgrmill/> (Sat, 20 Nov 1999)
  5. phosphorus questions
    by "Roger S. Miller" <rgrmill/> (Sun, 21 Nov 1999)
  6. phosphorus andn phosphorus control
    by "Roger S. Miller" <rgrmill/> (Tue, 23 Nov 1999)
  7. PO4 and Algae
    by "Dixon, Steven T. (BEn)" <stdixon/> (Mon, 21 Feb 2000)
  8. Bacteria, algae, and N:P
    by Thomas Barr <tcbiii/> (Wed, 10 Jan 2001)
  9. N:P
    by Paul Sears <psears/> (Thu, 11 Jan 2001)
  10. Bacteria algae and N:P
    by "Roger S. Miller" <rgrmill/> (Thu, 11 Jan 2001)
  11. RE: Seeking good phosphate test kit.
    by "Pearlscott, Mark HAMOH" <PearlscottM/> (Mon, 29 Jan 2001)
  12. Phosphate test kit
    by K9AUB/ (Mon, 29 Jan 2001)
  13. PO4 kits
    by Thomas Barr <tcbiii/> (Mon, 29 Jan 2001)
  14. PO4 source
    by "jay reeves" <JREEVES1/> (Thu, 7 Feb 2002)

phosphorus and phosphorus control

by "Roger S. Miller" <rgrmill/>
Date: Wed, 17 Nov 1999


Recently I spent some time pondering the role of phosphorus in aquariums.

We usually think of phosphorus more in terms of dissolved phosphate, but
(excepting the many biological forms) phosphorus actually occurs in
aquariums in four different forms:  dissolved inorganic phosphorus,
particulate inorganic phosphorus, dissolved organic phosphorus and
particulate organic phosphorus.  The inorganic forms are phosphates and
the organic forms are molecules derived from the decomposition of
biochemicals.  There are important interconversions between the different

What follows is a sad attempt at ascii art that tries to illustrate some
of the more important aspects of the system.

                    Water                 Water
                    changes               changes
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                    ----------           ----------
Water change        Dissolved <<<<<<<<<< Dissolved     
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic   <<<<<<<< Fish waste
Additives           Phosphorus           Phosphorus
                    ---------- __        ----------
                      vv       |\
                      vv         \
                      vv          \
                      vv           \
                      vv            \
                      vv             \
                      vv              \
                      vv               \
                      vv                \
                    -----------          -----------
 Fish Food          Particulate          Particulate        Fish Food
 Fish Waste >>>>>>> Inorganic            Organic    <<<<<<< Fish Waste
                    Phosphorus           Phosphorus
                    -----------          -----------
                      vv                   vv
                      vv                   vv
                      vv                   vv
                      vv                   vv 
                    Removal              Removal

The sketch shows each of the four major types of phosphorus, the major
sources for each type, the interconversions between them and the possible
fates of each form.  For simplicity, biological uptake of phosphorus isn't
included in the sketch.  The pathways shown with double lines of symbols
are pathways that might be promoted to reduce problems with dissolved
phosphorus levels in an aquarium.  The pathways shown with single symbols
are pathways that might be supressed to reduce phosphorus problems.

This whole thing could be given a lot of words, but I'm going to try
to limit myself to a brief description.

*Dissolved inorganic phoshorus* is biologically available and essential to
plants and algae.  It is mostly dissolved phosphates.  It enters the
aquarium from tap water, sometimes from water treatments like pH buffers
and as inorganic phosphorus wasted through the gills and kidneys of
animals.  It is removed from the aquarium by water changes.  It is
produced in aquariums from organic phosphorus by bacterial and algal
phosphatase activity.  It is converted to particulate inorganic phosphorus
by sorption and precipitation.

*Particulate inorganic phoshorus* is mostly not biologically available. It
is phosphate associated with phosphate minerals and adsorbed on metal
hydroxides and other solids in the aquarium.  It enters the aquarium
mostly in fish food and animal feces and it can be removed by siphoning
out detritus and cleaning filter media. It is formed within an aquarium by
sorption and precipitation of dissolved inorganic phosphorus.

*Dissolved organic phosphorus* is biologically available to bacteria and
possibly to algae, but I haven't found references saying it is available
to plants.  It enters the aquarium from animal waste. It can be removed by
water changes.  Bacterial and algal phosphatase activity convert dissolved
organic phosphorus to phosphates.

*Particulate organic phosphorus* isn't available to plants and algae, but
is available to animals.  It enters the aquarium as plant detritus, fish
food and feces.  It can be removed by siphoning and filter cleaning.  It
is converted to dissolved phosphates by phosphatase activity.  The
phosphatase activity is partly due to detritivores but also to bacteria
and possibly algae.

I wrote a little spreadsheet program to balance the reactions and provide
estimates for steady-state concentrations of each component with various
estimates of the reaction rates.  If there is interest then in later
letters I can try to summarize what that all seems to mean to controlling
phosphorus levels in an aquarium.

Roger Miller

phosphorus and phosphorus control (2)

by "Roger S. Miller" <rgrmill/>
Date: Thu, 18 Nov 1999


Well, I guess there was enough interest after the first letter to justify
my going on.  Too bad it got this long...

First to David Youngker's comment about my purpose...

My goal is to help people find ways to control excessive algae growth
caused by high phosphorus levels.  I think it should be possible to
control those problems with good tank-management habits -- without large
investments in chemical test kits, special filter media or a minor in

And on to other things...

Yesterday I foisted this diagram on the list:

                    Water                 Water
                    changes               changes
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                    ----------           ----------
Water change        Dissolved <<<<<<<<<< Dissolved     
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic   <<<<<<<< Fish waste
Additives           Phosphorus           Phosphorus
                    ---------- __        ----------
                      vv       |\
                      vv         \
                      vv          \
                      vv           \
                      vv            \
                      vv             \
                      vv              \
                      vv               \
                      vv                \
                    -----------          -----------
 Fish Food          Particulate          Particulate        Fish Food
 Fish Waste >>>>>>> Inorganic            Organic    <<<<<<< Fish Waste
                    Phosphorus           Phosphorus
                    -----------          -----------
                      vv                   vv
                      vv                   vv
                      vv                   vv
                      vv                   vv 
                    Removal              Removal

The most evident controls on all the phosphorus types is to reduce the
phosphorus sources (fish food, fish wastes, and phosphates in tap water)
and increase removal of phosphorus with larger or more frequent water
changes and more effective removal of phosphorus-containing sediments.

*Water changes* seem like a great way to keep dissolved forms of
phosphorus from building up in a tank, but that may not be as effective as
it seems at first blush.  In my case, for instance, I do 15% water changes
in every tank, every week.  Along with 15% of the water, the water changes
remove only 15% of the phosphorus. Commonly recommended change schedules
like 20-25% every two weeks (equivalent to 10% to 12.5% per week) are even
less effective.  Still, 10% is better than nothing and for most of us
practical limits on time and energy keep our water changes at this sort of
low level.

People who have significant phosphate in their water supplies have a
different problem.  For them, water changes may actually increase rather
than decrease the phosphorus supply in their aquarium.  For those people
smaller water changes may be better.  They may need to use water run
through a commercial phosphate-removing media like activated alumina, or
purified (ultrafiltered or deionized) water.

*Sediment removal* - usually done just by siphoning off detritus and
cleaning filters - is another great-sounding ploy, but here too there are
some snags.  In a bare tank it's easy to remove all the visible detritus
during the periodic cleanings and to wash all of the built up material
from filters.  It's a lot harder to get the detritus out of a heavily
planted tank, but even with heavy planting it should be possible to remove
more than 50% of the detritus at each cleaning.

Most of us clean our tanks and filters on a periodic basis.  Between
cleanings the accumulated organic particulates can be attacked by bacteria
and release dissolved phosphorus.  By the time we remove the accumulated
particulates we may be getting only a small fraction of the phosphorus
that it originally contained.

On general principles filters should be cleaned as frequently as possible
to limit the release of dissolved phosphorus.  Unfortunately most of the
popular aquarium filters are difficult enough to get into that daily
cleanings are out of the question.  For most of us, daily siphoning of
detritus is also out of the question.

Both cleaning and water changes provide important controls on the
accumulation of phosphorus in the aquarium but they are of limited
effectiveness.  There are a few additional options for phoshorus removal
that depend on conversions within the aquarium and I'll describe those
when I get around to writing about those conversions.

That brings us to the options for controlling the additions of phosphorus
to our aquariums.  It's pretty easy to avoid adding phosphate fertilizers
or other phosphorus-containing chemicals to the tank.  It's more
difficult, but possible, to avoid adding phosphorus to the tank in the
water supply that's used for water changes.  Once those inputs are
controlled, we're still left with a large influx of phosphorus.  That
influx is from feeding, either directly as uneaten food or indirectly as

*Feeding* can be controlled in two different ways; in quantity and in
quality.  Controlling the quantity of feedings is pretty straightforward.  
Many of us (including myself) probably feed more than is necessary for
maintaining healthy adult fish.  Since feeding is the major source of
phosphorus for the tank, reducing the feeding can have a proportionate
effect on the phosphorus level in the tank.  Decreasing the feeding rate
by 30% can reduce the phosphate level in the tank by 30%, a 50% decrease
in feeding can halve the phosphate levels, and so on.  Most of us are
probably pretty happy with the way we're feeding our fish and may not be
comfortable with large reductions, so there's a limit to the control that
can be attained this way.  Probably the most comfortable way to reduce
feedings is to reduce the fish population in a planted tank.  That lets
you reduce the feeding without running the risk of stressing your favorite

Controlling the quality of the feed is not as straightforward.  The main
idea is to use food with a lower phosphorus content.  Many prepared foods
contain more phosphorus than fish need and not all (perhaps less than
half) of the phosphorus is even in a form the fish can use.  Look for
foods with lower phosphorus content that are formulated for the specific
needs of your fish.  You can even do this with live and fresh foods if you
can find the phosphorus content of the foods you're using.

There is also an approach for those who want to force phosphorus to be
more growth-limiting than nitrogen.  Feed your fish a diet with a ratio of
at least 7:1 between available nitrogen content and available phosphorus
content.  At that ratio, phosphorus should be in short supply compared to

It's fairly easy to check this with prepared foods.  The total nitrogen
content can be estimated by dividing the crude protein content by 6.25.  
Probably most of the nitrogen content will be biologically available.  
Total phosphorus content should be listed on the packaging.  As little as
half the phosphorus content may be biologically available, but it's safer
to assume (at least initially) that all of it will be biologically
available.  If you use a diet that is 45% protein (for instance), then it
will contain 45/6.25=7.2% nitrogen.  In order for phosphorus to be
growth-limiting the phosphorus content of the food should be about 1/7th
that amount, or 1% of the total.  If the actual phosphorus content of the
diet is 1% or less, then phosphorus should be growth limiting.  This is
not as easy when using live or fresh foods but as long as you can find the
protein and phosphorus contents of the food you can use the method.


Hopefully that answered some of the initial questions left after my first
letter.  In another letter I'll try to describe some of the
interconversions between forms of phosphorus in an aquarium.  I'll even
give thought to how they might be used to control phosphate levels in an

Roger Miller

phosphorus and phosphorus control (3)

by "Roger S. Miller" <rgrmill/>
Date: Sat, 20 Nov 1999


Remember this?

                    Water                 Water
                    changes               changes
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                    ----------           ----------
Water change        Dissolved <<<<<<<<<< Dissolved     
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic   <<<<<<<< Fish waste
Additives           Phosphorus           Phosphorus
                    ---------- __        ----------
                      vv       |\
                      vv         \
                      vv          \
                      vv           \
                      vv            \
                      vv             \
                      vv              \
                      vv               \
                      vv                \
                    -----------          -----------
 Fish Food          Particulate          Particulate        Fish Food
 Fish Waste >>>>>>> Inorganic            Organic    <<<<<<< Fish Waste
                    Phosphorus           Phosphorus
                    -----------          -----------
                      vv                   vv
                      vv                   vv
                      vv                   vv
                      vv                   vv 
                    Removal              Removal

Two days ago I described the forms of phosphorus shown in the diagram and
listed the mechanisms that add and remove each form.  Yesterday I
described the major routes in and out of the aquarium and some means of
using those routes to control the phosphorus levels.  Today I guess I'll
write about the reactions within the aquarium and how they might be used
to help control the concentration of dissolved forms of phosphorus.

The terms for different forms of phosphorus are repeated frequently in
this discussion. I'm going to use some abbreviations to keep the text from
becoming far too cumbersome:

   DPi is dissolved inorganic phosphorus
   PPi is particulate inorganic phosphorus
   DPo is dissolved organic phosphorus
   PPo is particulate organic phosphorus

*Phosphatase* is an enzyme that bacteria use to convert DPo and PPo to
DPi.  Some algae may also have that ability.  Alkaline phosphatase - which
works when the pH is near and above 7 - is the most well documented form
of phosphatase. There is also an acid phosphatase that doesn't seem to be
as well documented.  Phosphatase activity increases with the size of the
bacterial population; activity is promoted by oxygen and suppressed by
dissolved phosphate.

Bacteria typically produce more phosphate than they consume, and that can
lead to an excess of phosphate in the water.

The phosphatase-catalyzed reaction from PPo to DPi should be discouraged
to prevent production of excess dissolved phosphate.  If the reaction can
be slowed down then phosphorus will remain in the detritus where it can be
more easily siphoned off before it causes problems.

The reaction from DPo to DPi is a somewhat different matter.  My reading
indicates that some algae may be able to utilize DPo but plants cannot.  
If that's true then DPi - which can be utilized by plants and algae - are
preferred to DPo, so the conversion of DPo to DPi should be encouraged.

How do we suppress the phosphatase-catalysed reaction from PPo to DPi
while we still allow the phosphatase catalyzed reaction from DPo to DPi?  
I don't know, but algal scrubbers come to mind.  My impression is that
this is probably a fairly fast conversion that would be difficult to

I can speculate about how we might reduce the rate of
phosphatase-catalyzed reactions, but I don't have any real data.  The
conditions that promote high levels of phosphatase activity and so cause
rapid release of phosphorus from detritus are exactly the conditions found
in aquarium filters. In a filter, organic particulates are trapped in a
constant flow of aerated water where a large population of bacteria can
act on the detritus.  The flowing water also flushes the phosphate
released by the reaction, which otherwise could suppress further
phosphatase activity.  So removing a filter should reduce the phosphatase

I keep unfiltered tanks.  I think in retrospect that pulling the filters
off my tanks helped control phosphate levels, but I can't substantiate
that.  Has anyone measured phosphates over a period of time after removing
the filter from a planted tank?  Or observed changes in a tank after
removing the filter that would suggest a change in phosphate levels?

*Precipitation and sorption* convert DPi to biologically unavailable PPi.  
This is a group of reactions that in nature are largely responsible for
keeping phosphorus in a growth-limiting role.

DPi combines with calcium to precipitate a number of insoluble solids.  
Dissolved calcium (hardness) and pH near and above 7 promote formation of
the solids.  The solids don't precipitate readily and may not form at all
without a suitable substrate or "seed" to grow on.  Industrial and
municipal-scale phosphorus removal systems use several different kinds of
seeds.  One system uses plain sand as a seed for forming the
phosphorus-bearing solids, other processes use calcium carbonate (which
isn't stable under common conditions in planted tanks) or one of several
kinds of calcium phosphates.  Dissolved organic compounds like tannins can
foul the seed surfaces and slow down the precipitation of calcium

Very soft water and water with a low pH should be prone to elevated
phosphate levels because the DPi won't precipitate. If you have excess
phosphate in a tank with soft water or pH below 7 then you might be able
to increase the calcium content and pH of your water and add a fresh seed
material to your filter media to promote precipitation of calcium
phosphate solids.

Dissolved phosphates also have a strong tendency to stick to many kinds of
surfaces.  The generic term for this process is sorption.  Phosphorus is
very stongly associated with surfaces on ferric oxyhydroxides and aluminum
hydroxides.  Ferric oxyhydroxides are common in nature.  In most soils -
including aquatic soils - it is often sorption on ferric compounds that
keeps phosphorus in place and out of the water.

Low levels of dissolved phosphorus can be removed from water by dosing
with a ferrous chloride solution.  The ferrous iron oxidizes to ferric
iron and precipitates as the oxyhydroxides.  DPi is strongly associated
with the iron and settles out with it.  That process also lowers pH and
alkalinity.  DPi may also attach to laterite and other soil substrates
that contain ferric oxyhydroxides.

Several companies sell filter materials on the aquarium market that use
the same mechanism to pull phosphate out of the water.  I think that those
products use activated aluminum hydroxide.  In my experience, the products
work, but often not for long and they can be fairly expensive to use on a
regular basis.  They attach a lot of compounds other than phosphate.  
Those other compounds (particularly silica and some organics) can pretty
quickly foul the material and ruin it's capacity to remove phosphate.  I
suspect that phosphate-adsorbing products would be more effective when
used as a pre-treatment to remove phosphorus from tap water than they are
as an aquarium filter medium.


In my last letter on this topic I'll try to summarize all this and use the
results of a simple spreadsheet model to assess the relative effectiveness
of different control methods.

Roger Miller

phosphorus and phosphorus control (4)

by "Roger S. Miller" <rgrmill/>
Date: Sat, 20 Nov 1999


Tests with a simple model of phosphorus states in aquariums show that
reducing feeding rates is the most effective means of reducing phosphate
concentrations.  The second most effective ploy is to encourage higher
rates of phosphate precipitation or sorption and the third most effective
ploy is to increase water changes.

Here it is the familiar diagram again, this time for the last installment:

                    Water                 Water
                    changes               changes
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                      ^^                    ^^
                    ----------           ----------
Water change        Dissolved <<<<<<<<<< Dissolved     
Fish waste >>>>>>>> Inorganic <<<<<<<<<< Organic   <<<<<<<< Fish waste
Additives           Phosphorus           Phosphorus
                    ---------- __        ----------
                      vv       |\
                      vv         \
                      vv          \
                      vv           \
                      vv            \
                      vv             \
                      vv              \
                      vv               \
                      vv                \
                    -----------          -----------
 Fish Food          Particulate          Particulate        Fish Food
 Fish Waste >>>>>>> Inorganic            Organic    <<<<<<< Fish Waste
                    Phosphorus           Phosphorus
                    -----------          -----------
                      vv                   vv
                      vv                   vv
                      vv                   vv
                      vv                   vv 
                    Removal              Removal

In my three previous letters I described the major elements in this
diagram, discussed the terms that add and remove phosphorus from the
system, and most recently described the interactions between the forms of
phosphorus within the aquarium.  In this last letter I hope to present a
simple model of the system and describe the model's results pertinent to
controlling dissolved phosphorus levels in aquariums.

Very briefly, the model consists of four equations, each representing the
mass balance for each of the four forms of phosphorus in the system.  The
arrowed lines in the diagram above are represented in the model by a term
in one or two of the equations.  The resulting system of equations can be
solved for the concentration of each of the four phosphorus forms as a
function of the input of fish food and fish wastes, with the terms in the
equation representing the rates of water changes, sediment removal,
phosphatase activity and precipitation or sorption.

Many of the coefficients in the model depend on details of an aquarium
setup; maintenance, food composition, species selection and so on.  Some
of the details are fairly insignificant, and others have duplicative
effects.  The model has 10 coefficients. I varied two of the coefficients
to create three different test cases that cover a range of possible
aquarium conditions, Here are all 10 of the model coefficients, and the
values used for each of the three test cases.

                                    case 1     case 2      case 3

Feeding rate
   grams phosphorus/week             0.02       same         same

Phosphorus digestibility
   percent                           50%        same         same

proportion of inorganic phosphorus
   in undigestible fraction          80%        same         same

proportion of inorganic phosphorus
   in digestible fraction            20%        same         same

precipitation rate for
   dissolved inorganic phosphorus
   percent per week                  67%        100%          25%

mineralization rate for
   dissolved organic phosphorus
   percent per week                 100%        same         same

phosphatase activity for
   particulate organic phosphorus
   percent per week                  50%        10%          67%

water change rate
   percent per week                  15%        same         same

cleaning efficiency
   percent per week                  67%        same         same

resulting phosphate
   concentration in a
   55 gallon tank                  0.21 ppm     0.12 ppm     0.42 ppm

I used the cases to test 5 coefficients to see which ones could be used to
most effectively control phosphate concentrations.  I measured the
effectiveness by calculating the ratio between the percent change in the
phosphate concentration and the percent change in the coefficient.  Here
are the results for each model and each tested coefficient:

                                    case 1     case 2     case 3

feeding rate                          1.0        1.0        1.0

precipitation rate                   -.8073    -.8622     -.6098      

water change rate                    -.2713    -.2261     -.4540

cleaning efficiency                  -.0496    -.0243     -.0499

phosphatase activity
  for particulate organic phosphorus  .0494     .0242      .0497 

There aren't very many suprises here.  In all cases, reducing the feeding
rate is the most effective way to decrease phosphate concentrations.  The
second most effective alternative is to increase the rate of precipitation
or sorption of dissolved phosphate.  It's interesting that among the three
cases, precipitation is less effective for case 3, where the phosphate
concentration is highest.  Increasing water changes was the third most
effective ploy.  Again, case 3 is interesting because in the case with the
highest phosphate concentration water changes are relatively more

The effectiveness of changing cleaning efficiency and phosphatase activity
were substantially lower than for the first three tactics.  Oddly, in
every case improvements in the cleaning efficiency and decreases in the
phosphatase activity had nearly identical effects.

I hope that those of you who managed to plow through all this found
it useful and thought-provoking.

Roger Miller

phosphorus questions

by "Roger S. Miller" <rgrmill/>
Date: Sun, 21 Nov 1999

On Sun, 21 Nov 1999, Stephen Lindsay wrote:
> a. I agree as previously stated on the list that smaller water changes made more
>     often will probably result in less removal of phosphorus than larger changes
>     made with longer intervals, because of the dilution in the former.

This depends on the relative size of the small, frequent changes and the
larger, less frequent changes.  15% water changes every week will remove
more than 25% water changes every two weeks.  15% water changes every week
will not remove as much as 30% water changes every two weeks.  But the
difference in this latter case isn't significant enough for me to worry
> b. Why do you suggest suppressing the phosphatase-catalysed reaction from
>     PPo to PPi apparently in preference to the similar reaction of DPo to PPi, since:
>    "some algae may be able to utilize DPo but plants cannot"? You then suggest
>    using algae to possibly aid that suppression.

First, I'm not sure I understand the question.  Second, the product in
both of these reactions is DPi, not PPi.  I suggest suppressing the
reaction from PPo to DPi to avoid the production of DPi.  I suggest
encouraging the reaction from DPo to DPi because DPo appears to be nothing
but algae food.  As DPi, at least plants will compete for the phosphorus.

The algae scrubber idea assumed that the scrubber could be built in such a
way that it didn't trap particulates, hence would not effect the PPo to
DPi reaction.
> c .When DPi attaches to laterite (for example), should this then be replaced at
>     appropriate intervals in the substrate as it becomes saturated? And could any
>     reverse reactions take place (or even in a filter)?

I wouldn't suggest rebuilding a substrate to replace the laterite, even if
it does become saturated with phosphorus.  In a filter the media should be
replaced regularly.  In fact, the sorption media probably shouldn't be
left in the filter full time because it will foul and lose effectiveness
fairly quickly.
> d.  Are anaerobic bacteria capable of acting on organic particulates from detritus
>     where it becomes trapped in a slower flow of water situation as in a trickle filter?
>     (And so solve the "flush" case you mentioned).

Anaerobic bacteria can work on particulates under low flow conditons.  In
fact, obligate anaerobes have been found living inside organic particles
suspended in fully aerated water.  However, reactions catalysed by
anaerobic bacteria are generally slower than by aerobic bacteria. Under
low flow conditions whatever phosphate is freed in an anaerobic reaction
should remain near the point of the catalysis and suppress further

Roger Miller

phosphorus andn phosphorus control

by "Roger S. Miller" <rgrmill/>
Date: Tue, 23 Nov 1999

On Tue, 23 Nov 1999, Elliot Williams wrote:

> So what foods are low in phosphorus but not too low?

There are actually two issues here.  The first is phosphorus content
and the second is phosphorus digestibility.


I did a quick check about my house and found that Tetra was the only make
of fishfood in the house (I found 3) that listed phosphorus content.  Of
their several products, the new TetraMin Pro line contained higher
protein, lower minimum phosphorus and higher fat content then their other
products. Still not low in phosphorus, but lower.

I suspect that phosphorus content for other brands of prepared fish foods
can be obtained from the manufacturer even if it isn't posted on the
container.  Phosphorus in other forms of food (including live and fresh
foods) are probably available either from the supplier or from published

In salmonid culture high fat content is important for controlling nutrient
pollution from feeding growing stock.  The principle is very simple; fish
have a minimum calorie intake required for growth and if they can't get if
from fats then they burn proteins and other calorie sources for energy and
more of the food's nutrient content is wasted to the water.  I suspect
this principle will work in grow-out aquariums too.  It probably won't
effect aquariums with a mature, non-breeding fish population.

I'd like to find some information on phosphorus requirements for fish.  I
know it's out there.  I'm pretty sure that the total phosphorus content in
most foods far exceeds the fish's requirements, partly because of the
digestibility issue below.


Foods used in aquaculture can be rated for the percent of total phosphorus
(or any other nutrient) content that can be used by the fish.  In those
cases, the foods are designed for and fed to a single species of fish and
things are relatively simple.  In aquariums we deal with mixed populations
of fish and generally we support some scavengers and detritivores, so the
situation isn't as simple.

The largest part of most prepared foods is fish meal.  The fish meal has a
high phosphorus content but a large part of that is in the ash fraction of
the food.  It consists of ground up bone and scales that aren't digestible
to most fish.  Yet another part of the total phosphorus content is part of
the vegetable matter and it is undigestible to most fish.

The undigestible portion of the food ends up in the feces and becomes part
of the particulate fraction.  If a tank is regularly cleaned then at least
some of the undigestible fraction will be removed before it becomes
available to plants and algae.

The digestible part can be used for growth or egg production.  But in
mature non-breeding fish it can only be used for maintenance.  In
maintenance I think that any phosphorus that the fish use will be balanced
in the waste stream by phosphorus that they eliminate through their gills
and kidneys in dissolved organic or inorganic forms.  Those forms are
immediately available to plants and algae.  They're unavoidable if
you're maintaining fish, but it would be good to minimize the input.


So from all this, there appear to be a couple things that can be done.
First and foremost is to feed the little beggers less.  Second is to be
sure that the food you feed is appropriate for the fish you keep, with no
more phosphorus than they need and with that phosphorus in a form that's
available to the fish.  It might even help if you keep some growing and/or
breeding fish in the tank as long as you feed them with a sufficiently
high-calorie diet.  Live bearers seem to be great for that.

Roger Miller

PO4 and Algae

by "Dixon, Steven T. (BEn)" <stdixon/>
Date: Mon, 21 Feb 2000

Roger wrote about a lack of algae growth under seemingly adequate macro and
micronutrient conditions:

"Rather than restate the question yet again (which might make this thread
sound a lot like attorney's questioning a difficult witness), I'll go ahead
and propose three possibilities:
1)	The algae population in your tank isn't well-adapted to use the PO4
under the conditions of light, water hardness and so on that your tank
provides.  This would probably be a temporary state of grace because new
algae varieties are inevitably introduced.  I think this is least likely of
my three options.
2)	The algae population is being kept in check by grazing.  This would
mean by microscopic grazers (per James Purchase's recent quote from Craig
Bingman) or more normal grazing by SAEs, etc.  In this case, if you dosed
too much PO4 you might get growth rates that exceed the appetite of your
grazers.  I don't have any way to assess the likelihood of this option, but
I really like it anyway; it has such an ecosystem "feel" to it.
3)	Algae growth is actually being controlled by iron (or perhaps,
manganese). The observed growth could occur in the transition from
phosphate-limited growth to iron-limited growth, or it could be a response
to "luxury" levels of phosphorus.  I think this is probably the most likely
of the options.  In order for iron to control algae growth (particularly in
Karen's case, where she doses to some fairly high iron levels) I think it's
necessary that the iron test kits overstate the amount of available iron;
the amount of available iron would have to be no more than a small fraction
of the measurable iron."

I don't think # 1 is likely.  For one, I have had these conditions in my
tanks for several months at a time.  #2 is attractive, but doesn't seem
plausible to me either.  My algae crew is fat and well-fed and happy.  Their
algae work is just a friendly pastime between feedings of brine shrimp,
blood worms, mosquito larvae, dry food, etc.  And there are less of them now
than at times when algae was a problem for me.  In addition, this crew has
been easily overwhelmed by algae problems associated with nitrate limitation
on repeated occasions a few years ago.  # 3 is definitely worth thought and
consideration.  But it's not iron.  I'm a careful student of iron.  I have
had intensive outbreaks of algae with less (actually much less) iron than I
presently have in either of my tanks.   (A word of caution about reported
iron levels.  Even the best hobbyist kits (LaMotte and Hach, I have both)
struggle a bit to pull out the chelated iron we commonly use in our
micronutrient solutions.  I continue to believe the cheap kits are
essentially worthless and actually often yield "false positives" if you

I'm interested in the conversations I've seen about manganese.  It's a
nutrient I want to learn about.  While I believe I understand the essential
logic of the view that the higher plants must be pulling some nutrient or
other to a "limited" condition to out compete algae (Liebniz law), my
experience with nasty algae outbreaks only in nitrate limited conditions,
and superb stable growth conditions with seemingly fully adequate nutrition
levels with very little bothersome algae growth, has me wondering whether
we've got all the pieces on the chess table.  Why wouldn't algae grow well
under conditions of adequate general nutrition?  Very odd idea, isn't it?  I
suppose that's Roger's point! :-)

I like the way Karen characterized the nitrate/phosphate limitation issue in
her recent note.  I might go one step farther.  I'm not actually sure that
limiting, or nearly limiting, phosphate is the important issue.  I'm rather
more certain that NOT allowing nitrate to become the limiting factor is the
important point.  I think if we studied this carefully, we might make this
type of revision to Paul and Kevin's original idea.

And by the way,  I stopped by Tom Barr's house in Marin County, CA yesterday
and was very surprised to find 0.8 ppm phosphate in his large tank, and 1
ppm phosphate in his water supply.  Many of us in the Bay Area have observed
that Marin water seems to have some magic about it.  The aquatic gardeners
can often do quite well without the difficulties that perplex their brothers
and sisters just over the bridge in San Francisco.  FWIW, for those of you
who know Tom and his tanks, he is rarely troubled with algae problems.  

So let's keep noodling on this!

Steve Dixon 

Bacteria, algae, and N:P

by Thomas Barr <tcbiii/>
Date: Wed, 10 Jan 2001

I have been poking through some  Aquatic Microbiology mag's I saw in the
reference library and it seems there are many, many species of algicial
bacteria. Many species attack cyano's(old= blue green algae) and a few other
forms of algae but do not attack the Greens often. At least there's no
supporting evidence that I've found so far. I intend to look further into
this issue though. There's been some work but there's much yet to be done.
I have felt that the soil/substrate plays the most critical role in both
nutrients and bacterial cycling of these nutrients but this algicial
behavior with cyano's seems relevant to our tanks and could be a solution
even without using antibiotics. Perhaps there are others than can target
specific algae. In many tanks, we find algae cannot exist/persist/compete
even though the nutrient are available to them. Bacterial actions/presence
may be the key.
I'll see what I can dig up out of the substrate later:)

Also, I found compelling evidence for not letting your N:P ratio fall below
16:1 for aquatic plants. Neat stuff. So if you have a NO3 of 5ppm your P
would/should not fall below 0.3ppm or so. The optimum seemed to be about
10:1 or so of the studies I looked at. So .5ppm for a NO3 of 5ppm. This
might explain why at 1.2 ppm of P and my higher NO3(10-15ppm or so) that my
plants did well. Steve Dixon did this lower NO3 at 2-5ppm with a pulse of P
to about .2ppm or so. He had good results and so did I. We both had a
similarly close ratio. I have been doing to the lower end ratios these
days(pulses of P with lower NO3 2-5ppm or so) I want to find out how far
this ratio can go. What is the upper limit with a set of CO2 levels/light
values and excess Fe, K+ and trace elements without algae build up?
I'll need to get a new probe for my tank so I'll be doing this once it

Tom Barr 


by Paul Sears <psears/>
Date: Thu, 11 Jan 2001

	I don't think the idea has been to drive the phosphate concentration
as low as possible.  I think the idea now is to avoid running out of anything
else (any other nutrient that the plants need).  Quite a few of us have posted
about deliberately adding phosphate to our tanks to get things going.
When there is sufficient of all the other nutrients there, phosphate
additions are very useful.

- -- 
Paul Sears        Ottawa, Canada

Bacteria algae and N:P

by "Roger S. Miller" <rgrmill/>
Date: Thu, 11 Jan 2001

On Thu, 11 Jan 2001, Tom Barr wrote:

> Also, I found compelling evidence for not letting your N:P ratio fall below
> 16:1 for aquatic plants. Neat stuff. So if you have a NO3 of 5ppm your P
> would/should not fall below 0.3ppm or so.

This number has been around for a while (25 years or so, anyway) as the
value above which aquatic ecosystems tend to be nitrogen limited and below
which they tend to be phosphorus limited.  Be careful, though, because the
ratio is for N and P concentrations, not NO3 and PO4 concentrations.

A NO3:PO4 ratio of 23:1 is equivalent to an N:P ratio of 16:1, as long as
N is available largely as NO3 and P is available largely as PO4.  If
you want nitrogen-limited conditions and have 5 ppm NO3, then you
shouldn't let PO4 exceed about 0.2 ppm.

Of course you may get erratic results when you start with a value that was
derived as an approximate rule-of-thumb for ecosystems and apply it to an

Roger Miller

RE: Seeking good phosphate test kit.

by "Pearlscott, Mark HAMOH" <PearlscottM/>
Date: Mon, 29 Jan 2001

> Date: Mon, 29 Jan 2001 11:41:30 -0800
> From: Ted Backman <>
> Subject: Seeking good phosphate test kit.
> Could someone recommend one?
> also, I'd be interested in hearing what people thought about 
> the various
> phosphate absorption products (pillows, mats, solutions, etc 
> ) that are on
> the market, and wheather they're appropriate for planted tanks or not.

Aquarium Pharm. makes a PO4 kit that is decent.  It was only about $10.
Easy to read scale.  Reads from 0-1mg/L and 1-10mg/L.  Decent for the price.
I'm sure Hach Chemical has a better kit, but it will cost a little more.

I can't recommend an absorption product better than plants.  It's possible
to shift your nutrients so that P is the limiting nutrient, and is
un-readable in test kits.

Mark P.

Phosphate test kit

by K9AUB/
Date: Mon, 29 Jan 2001

> Could someone recommend one?

You will be hard pressed to find one superior to the Seachem. It's a dandy.
>  also, I'd be interested in hearing what people thought about the various
>  phosphate absorption products (pillows, mats, solutions, etc ) that are on
>  the market, and wheather they're appropriate for planted tanks or not.

This question came up a couple of weeks ago, and everyone had a different 
answer.  All the phosphate absorbing products made from "white granules" are 
aluminum oxide.  All the products with "black powder" are using iron oxide.  
I like the Phos-Zorb pillow (white granules) simply because it comes in a 
ready-to-go pouch.  Plants also absorb phosphate, so a sturdy stand of 
growing plants should remove any phosphate you might have in the water.

PO4 kits

by Thomas Barr <tcbiii/>
Date: Mon, 29 Jan 2001

> Date: Mon, 29 Jan 2001 11:41:30 -0800
> From: Ted Backman <>
> Subject: Seeking good phosphate test kit.
> Could someone recommend one?
> also, I'd be interested in hearing what people thought about the various
> phosphate absorption products (pillows, mats, solutions, etc ) that are on
> the market, and wheather they're appropriate for planted tanks or not.

SeaChem is good and well priced. Hach makes a good one and Lamott makes a
good one also. Hach is the best but is pricey.
Get your plants to grow well and you will not need no PO4 absorption pillows
etc. My tap had loads of PO4. I have to add it these days. I watched my PO4
drop by .2ppm a day in a good running plant tank. NPK is what plants like to
grow well but it needs to be in balanced proportions, not absent.

Tom Barr

PO4 source

by "jay reeves" <JREEVES1/>
Date: Thu, 7 Feb 2002

Several folks have been asking about sources for PO4.  I know this has been
mentioned before, but is worth repeating.  I was given a couple of
tablespoons of Seachem Neutral Regulator, which contains phosphate as a
buffer) by a fellow aquarist who uses it as a PO4 source.  I mix 2 teaspoons
in 300 ml H20 and dose in 5 - 20 ml quantities 2 - 3 times a week in a 75
gal.  With a LaMotte test kit levels go up to 0.5 ppm (occasionaly higher)
and to 0 within 2 -3 days.  Seachem products are probably more readily
available to folks than other phosphate sources (except fish food).

Jay Reeves

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