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The Airstone Debate

Contents:

  1. The Geat Airstone Rebuttal
    by 00cgbabbitt-at-leo.bsuvc.bsu.edu (5 Feb 93)
  2. The Geat Airstone Rebuttal
    by 00cgbabbitt-at-leo.bsuvc.bsu.edu (5 Feb 93)
  3. (LONG) Airstones versus Powerheads vis-a-vis CO2 (was: Re:
    by jimh-at-ultra.com (Jim Hurley) (5 Feb 93)
  4. Airstone anathema
    by "David W. Webb" <dwebb-at-ti.com> (Thu, 3 Oct 1996)
  5. RE: air at night
    by Michael Moncur <mgm/starlingtech.com> (Fri, 22 Oct 1999)
  6. Zapping scum
    by Dave Gomberg <gomberg/wcf.com> (Sun, 05 Dec 1999)

The Geat Airstone Rebuttal

by 00cgbabbitt-at-leo.bsuvc.bsu.edu
Date: 5 Feb 93


        Recently I posted my thoughts on the usage of airstones vs.
powerheads. 
I did this as a service to inform you on the gross misconception that
powerheads are the only means of powering your under-gravel filteration.  I
would like to follow up due to the great response of opposition I face.
        Although aquarists are familiar with measurements for water
temperature
and pH, very few hobbyists have any idea what the amount of dissolved oxygen
in their aquarium water is, or what it should be.  In fact, it has never
occured to many fishkeepers that the concenteration of oxygen in the water can
be a matter of concern.
        In lightly stocked tanks, the amount of dissolved oxygen is usually
not
a problem, but in tanks with lots of fish, things can be much different. 
Factors such as lack of sufficient aeration, unusually warm water temperatures
or power outages can cause significant difficulties because of dangerously low
levels of oxygen in the water.  Each factor alone can result in problems, and
in combination the situation becomes even worse.
        Good aeration means a strong flow of air bubbles rising to the
surface. 
This serves two purposes: 1) agitate the surface of the water for better
exchange of dissolved oxygen and carbon dioxide and 2) create internal
currents
in the tank to bring water at the bottom to the top.
        The warmer water becomes, the less dissolved oxygen it holds.  At
normal tank temperatures, perhaps 76 degrees Fahrenheit (24 degrees Celsius),
the ideal concentration of dissolved oxygen is 7.0 parts per million (ppm). 
When the water temperature reaches 85 degrees Fahrenheit (29 degrees Celsius),
however, the oxygen level can drop as low as 5.5 ppm or less.  Many fish begin
to have difficulties when the dissolved oxygen drops below 5.5 ppm.
        At normal tank temperatures, the type of filteration used can have a
substantial effect on the levels of dissolved oxygen.  Typical measurements
for
various setups can be as follows: airstone-driven undergravel filter-7.5 ppm;
powerhead-driven undergravel filter with supplemental airstones in tank-6.5
ppm; powerhead-driven undergravel filter with no airstones-5.5 ppm;
reverse-flow undergravel filter with supplemental airstones in tank-6.0 ppm;
reverse-flow undergravel filter without airstones in tank-4.5 ppm.
        Clearly, at higher than normal temperatures, some of these setups
would
not have sufficient dissolved oxygen.  With the use of one or more airstones,
however, the problem can be corrected.
        A power outage is the worst of all situations in heavily stocked
tanks,
particulary in warm weather, because the dissolved oxygen level plummets
quickly.  Within hours some species of fish will die from the lack of
sufficient oxygen in the water.
        Chronically low levels of dissolved oxygen have two major effects on
the tank.  The fish are under continual stress from the lack of sufficient
oxygen for respiration and metabolism, and the nitrifying bacteria that
provide
biological filtration cannot metabolize ammonia and nitrate as efficiently,
creating even more stress for the fish.
        I am by no means suggesting that you forget about powerheads
altogheter, I am mearly suggesting that you look at the facts! 

The Geat Airstone Rebuttal

by 00cgbabbitt-at-leo.bsuvc.bsu.edu
Date: 5 Feb 93


        Recently I posted my thoughts on the usage of airstones vs.
powerheads. 
I did this as a service to inform you on the gross misconception that
powerheads are the only means of powering your under-gravel filteration.  I
would like to follow up due to the great response of opposition I face.
        Although aquarists are familiar with measurements for water
temperature
and pH, very few hobbyists have any idea what the amount of dissolved oxygen
in their aquarium water is, or what it should be.  In fact, it has never
occured to many fishkeepers that the concenteration of oxygen in the water can
be a matter of concern.
        In lightly stocked tanks, the amount of dissolved oxygen is usually
not
a problem, but in tanks with lots of fish, things can be much different. 
Factors such as lack of sufficient aeration, unusually warm water temperatures
or power outages can cause significant difficulties because of dangerously low
levels of oxygen in the water.  Each factor alone can result in problems, and
in combination the situation becomes even worse.
        Good aeration means a strong flow of air bubbles rising to the
surface. 
This serves two purposes: 1) agitate the surface of the water for better
exchange of dissolved oxygen and carbon dioxide and 2) create internal
currents
in the tank to bring water at the bottom to the top.
        The warmer water becomes, the less dissolved oxygen it holds.  At
normal tank temperatures, perhaps 76 degrees Fahrenheit (24 degrees Celsius),
the ideal concentration of dissolved oxygen is 7.0 parts per million (ppm). 
When the water temperature reaches 85 degrees Fahrenheit (29 degrees Celsius),
however, the oxygen level can drop as low as 5.5 ppm or less.  Many fish begin
to have difficulties when the dissolved oxygen drops below 5.5 ppm.
        At normal tank temperatures, the type of filteration used can have a
substantial effect on the levels of dissolved oxygen.  Typical measurements
for
various setups can be as follows: airstone-driven undergravel filter-7.5 ppm;
powerhead-driven undergravel filter with supplemental airstones in tank-6.5
ppm; powerhead-driven undergravel filter with no airstones-5.5 ppm;
reverse-flow undergravel filter with supplemental airstones in tank-6.0 ppm;
reverse-flow undergravel filter without airstones in tank-4.5 ppm.
        Clearly, at higher than normal temperatures, some of these setups
would
not have sufficient dissolved oxygen.  With the use of one or more airstones,
however, the problem can be corrected.
        A power outage is the worst of all situations in heavily stocked
tanks,
particulary in warm weather, because the dissolved oxygen level plummets
quickly.  Within hours some species of fish will die from the lack of
sufficient oxygen in the water.
        Chronically low levels of dissolved oxygen have two major effects on
the tank.  The fish are under continual stress from the lack of sufficient
oxygen for respiration and metabolism, and the nitrifying bacteria that
provide
biological filtration cannot metabolize ammonia and nitrate as efficiently,
creating even more stress for the fish.
        I am by no means suggesting that you forget about powerheads
altogheter, I am mearly suggesting that you look at the facts! 

(LONG) Airstones versus Powerheads vis-a-vis CO2 (was: Re:

by jimh-at-ultra.com (Jim Hurley)
Date: 5 Feb 93

This talk of airstones versus powerheads and CO2 loss is interesting.

I just convinced my wife to switch from a powerhead in a 27g plant tank
to an air-driven sponge filter. I was sure it would help the plants.
It won't take very long for us to find out.

I have about 9 planted tanks, most doing really well. 7 use
Tetra Brilliant sponge filters. The bubble rate is fairly small
so I imagine the turbulence is very slight.

The sheer pressure at the impeller of a powerhead has got to result
in massive turbulent flow around the blades.

A while back I asked a question on sci.chem about turbulence and CO2
and got into a small e-mail discussions with some folks who understand
this. I'll cut and paste some of that discussion and insert it
at the end of this post. I'll remove the headers and name references
so as not to offend anyone as I don't have their permission to
post this.

Here was my posting on sci.chem dated 21-Sep-92:
>I have a question related to aquarium plant keeping.  It is common
>knowledge among aquarist that one must keep turbulence down to a
>minimum in order to maintain the highest possible levels of dissolved
>CO2.
>
>CO2 readily dissolves in water, far more so than O2, and I think that
>turbulence aids in the dissolution of O2.
>
>Can someone explain why turbulence has this effect?
>
>I am thinking mostly of dissolution of solids in water:  if I stir a
>teaspoon of sugar in water it will dissolve far faster if I stir more
>quickly. Why is it that this breaks down for gases, and CO2 in
>particular?
>
>Will CO2 dissolve into a tank from the air quicker if there is a mild
>surface convection flow? Or is it better to leave the surface
>motionless?
>
>As you can surmise, I'm not a chemist, and I don't recall this
>topic at all from my chemistry classes.


--------------
In response to your recent question about dissolved gases and the
effects of turbulence, I can only offer the following explanation,
based on my experience with the use of ultrasound to generate
turbulence in solutions.

If you "sonicate" a liquid (like in those cleaning baths they use
in jewelry stores, turbulence is created along with small bubbles
(a process called cavitation).  Gas which is dissolved in the liquid
rapidly diffuses into the bubbles and they grow in size (a process
called "rectified diffusion").  Large bubbles "float" to the top of
the liquid and are released into the atmosphere.  For this reason,
ultrasound is used to outgas large volumes of liquids.  I don't
know enough about aquariums to know if the fluid velocities are
high enough to cause much cavitation, but if so, I think that the
same situation would occur as during sonication.

----------

For a grain of salt in water, say, there will occur a
concentration gradient- higher near the surface of the grain and
falling off with distance into the solution. The dissolution of
the solid is dependent on the conc. grad.; stirring mixes the
conc. soln with the less conc. so more solid can then dissolve.

The same holds true for gases dissolved in water but also to be
considered is the effect of turbulence on local pressure-
cavitation may result which provides a new phase for
partitioning and as a result there may occur a slight loss of
dissolved gas where there is turbulence.
[...]
lots of small bubbles (of co2) will give the quickest attainment
of equilibrium concentration (co2 disproportionates in solution-
that's why more dissolves than o2).

--------------
--------------
[I (jimh) asked for a clarification of cavitation:]

> I take it that 'cavitation' is like a localized vacuum, 'pulling'
> out some dissolved gases? If that is true, wouldn't most gases
> tend to come out due to turbulence, but since the atmosphere is
> providing N2, O2, and to a much smaller extent, CO2, there will
> be a far greater amount of O2 in turbulent water than CO2.
> This would happen because the turbulence would form many
> small bubbles which would tend to dissolve after they
> leave the turbulent area.

not a vacuum but rather a region of gas phase as opposed to liquid phase
chemicals (gas, liq, or solid) will always tend to move from areas of locally 
high concentration to areas of locally low concentration- the driving force
for
this is called chemical potential

[...]

most of this discussion is based on thermodynamics and relates to the max 
possible  amount of change (into/out of solution say) and does not address the
rate at which that equilibrium will be achieved.
with respect to this the tendency of a gas to enter(leave) a solution is 
dependent on partial pressure (its local conc), T, H (henrys law constant-
i.e.
its solubility), etc. Most gases will tend to be removed by turbulence but not
all to the same degree- also on leaving the region of pressure change the
bubble
will tend to redissolve (and will given infinite time- thats thermo; in 
a real time period, subject to bubble loss in time t; some-all-part of the
bubble may redissolve- that's kinetics)

[...]

(CO2)aqueous per se may not exist (conventional wisdom is that it does not!)
It is convenient to talk about this hypothetical concentration

H = P(CO2)/(CO2)aq = constant for given P & T ...
but
CO2 in aq solution reacts rapidly to form HCO3- (bicarbonate) + H+ and
to a much lesser extent CO3= (carbonate)

K1 = (H+)(HCO3-)/(CO2)aq = (H+)(HCO3-)/[H*P(CO2)] = constant depending
  on temp., ionic strength....

K2= (H+)(CO3=)/(HCO3-) = constant depending on T, ionic strength...

These equations are determined from chemical thermodynamics and are readily
solved for given pH, P(CO2).... for the missing values (quite often
pH is fixed via a buffer)
e.g. solving for pure water at room T and atmospheric CO2 levels gives
pH= 5.6

see Stumm & Morgan, Aquatic Chemistry or Drever Chemistry of Natural Waters

--------------------------
--------------------------
[This was getting to deep for me so I (jimh) wrote back:]

> From what you write, I would guess that the best way to get maximal amounts
of
> CO2 to dissolve from the atmosphere would be to bubble a mist
> of fine air slowly through the water from the bottom of the tank.
> That would create gentle convection currents and the mist would 
> increase the exposed gaseous surface area in the water.

yes, and the max will be determined by the equations given above. It is
possible
to supersaturate the solution with more than the thermo. max. (e.g. soda pop!)
but given time equilibria will establish (e.g. open pop goes flat!).

--------------------
--------------------
[The conversation then drifted to CO2 injection and welding tanks:]

I (jimh) wrote:

> I'll have to study your reply carefully as it's a bit over my head
> upon first reading. I'll try to find these books at the library:
> 
> -> see Stumm & Morgan, Aquatic Chemistry or Drever Chemistry of Natural
Waters
> 
> I've been told that welding gas is quite pure. At any rate, I've been using
> it for about a year without any unfavorable consequences. Other's
> have used brewery CO2, but some say that welding tanks are even more pure.
> I don't know. I imagine the gas comes from the same processes that
> make liquid nitrogen - from air liquefaction, but maybe it's from
> a chemical process.

most likely made from limestone + acid; CO2 in the air is ~ 350ppmV
(i.e. 0.035% by volume...give or take; there is some variation with location)
welding CO2 need not be pure for it's application - welding O2 for example is
quite unsuitable for many lab applications
Keep an eye open for a ref book 'Matheson Gas Data Handbook' (or something
like
that!)- Matheson is a major provider of gas cylinders and the handbook
gives specifications for different grades (or for typical values perhaps
perhaps just their catalog would suffice?) 
You can get Matheson's address from the Thomas Registry (most lib's have it)
or you can contact your supplier for a data sheet (their required to provide
an MSDS on demand by law- it may list the purity of the gas you are buying)


[ I (jimh) did this. The MDS claimed `100% CO2' ]

> 
> filter intake. The major safety problem, for me, is suffocation
> if the tank explodes, 

some home 'smoke' alarms will respond to CO2 you might check around
however if the regulator is checked often for wear and corrosion it is 
probably pretty safe. The cylinder must be inspected prior to refill and 
retested ever so often I believe. Generally the cylinders remain the property
of the supplier and you just 'rent' them.

> 
> The pH of my tap is about 7.6  - 7.8 at equilibrium. The total dissolved
> CO2 is rather small at that pH. By injecting the CO2, not only can the
> pH be controlled (good for some fish), but the plant growth become
> luxurious. It's fairly common in advance aquaculture to do this.

I see your point. On the subject of pH control you might find 
'Process Modeling, Simulation and Control for Chem. Eng' by Luyben 
(McGraw-Hill) useful. Lots of examples using stirred tanks, FORTRAN code
etc.

> 
> I only use this elaborate scheme on my largest tank - a 100 gallon tank.
> I do have 7 other smaller tanks with plants - the initial question
> was more directed towards these.

fresh or salt water? I always wanted to keep jellyfish or 
a tide pool aquarium. I see your mail was routed via ames what part of the
country are you located?


> 
> It was interesting to study pH probe design and acid buffering
> concepts. Water chemistry, and in particular, aquarium chemistry
> is fascinating. I never knew so much was happening in a simple
> aquarium tank. I wish I paid more attention in my chemistry
> classes in high school - it was pretty boring at that time.
> 

for marine chemistry you may want to take a look at 'An Intro. to Marine
Biogeochemistry' by S. Libes (Wiley)- a little easier on those that
slept through high school chem  ;-)

> In aquaculture, most people have a good understanding of basic
> chemistry and biology, but there are a great deal of myths,
> anecdotal evidence, and superstition. I'm just trying to
> get a bit more chemical understanding.

Good!

-- 
Jim Hurley --> jimh-at-ultra.com  ...!ames!ultra!jimh  (408) 922-0100
Ultra Network Technologies / 101 Daggett Drive / San Jose CA 95134

Airstone anathema

by "David W. Webb" <dwebb-at-ti.com>
Date: Thu, 3 Oct 1996


CO2 dissolved in water is at least partially present in the form of
carbonic acid (CO2 + H2O -> HCO3 + H+).  Carbonic acid lowers the pH, and
is utilized directly by the plants, raising the pH as they remove the acid
from the water.
Aeration will help control the pH in this case by driving off CO2
concentrations greater than what you'd find at atmospheric equilibrium.
Since often a tank without aeration and with little surface-air interaction
will increase its CO2 levels at night when the plants aren't consuming CO2
(they actually produce a little CO2 at night) and the same tank will drop
in CO2 levels during the day because the plants will consume the CO2 in the
water, the pH tends to fluctuate.  
Automated CO2 injection systems turn on and off CO2 delivery based on the
pH of the water to control the pH.  Other non-automated CO2 injection
systems rely on alkaline buffering of the water to keep the pH relatively
stable as CO2 is added and consumed at unequal rates.  There is still a pH
swing, but not as much as would be there if the water had no alkalinity.

If you have only plants that can take their CO2 needs from bicarbonates,
(Ca(CO3)2 + H20 -> CaCO3 + HCO3 + OH-) they can grow quite happliy by
consuming carbonate alkalinity for their CO2 needs (much more slowly
though, because the reaction above requires energy).  The extra OH- ion
left over will cause the water to rise in pH, though.  If you change your
water regularly, this shouldn't be a problem.  In this type of tank, the
CO2 levels maintained in the water by aeration are quite sufficient.  The
required alkalinity for this type of tank will probably keep your pH at
around 8.0-8.5, however.

If you have plants that cannot break down bicarbonates to gather their CO2,
they will have a very difficult time competing in this type of tank, and in
my experience, you set yourself up for an algae problem.  This is where CO2
conservation/injection techniques can come in particularly handy.  They
also work quite well on tanks that have carbonate-consuming plants.

Dupla's materials on aquatic plants suggest that many aquatic plants come
from streams with carbonic acid-rich springs exiting into the streams.
They also suggest that stream areas directly below rapids and other highly
aerating natural occurances generally don't grow plants quite as rapidly as
areas with less surface-air interaction.  My own observations concur with
this, both in the wild and in the aquarium.  I've found that CO2 injection
in my tanks can be quite useful in controlling algae outbreaks if I'm
keeping the rest of the nutrients at correct levels.

David W. Webb           Enterprise Computing
Texas Instruments Inc.  Dallas, TX USA
(972) 575-3443 (voice)  MSGID:       DAWB
(972) 575-4853 (fax)    Internet:    dwebb-at-ti.com
(972) 581-2380 (pager)  Text Pager:  dwebb-at-ti.com Subj:PAGE


RE: air at night

by Michael Moncur <mgm/starlingtech.com>
Date: Fri, 22 Oct 1999

I use an airstone at night in my CO2-injected tank. This adds O2, mostly 
via surface turbulence, and drives off much of the CO2. Here are the pros 
and cons of this approach as I see them:

PRO: Fish never suffer from oxygen deprivation. Fish seem healthier and 
happier.

CON: Expense (air pump and timer). Noise (air pump and diffuser). CO2 is 
driven off and wasted. The loss of CO2 causes a pH increase that may stress 
the fish.

I think this is a controversial practice; many people say that with healthy 
plants you never need to worry about oxygen. I've found this to be true, 
but with a major exception: if anything goes wrong with the plants, the 
fish can suffer due to lack of O2. A major water change, an algae or 
nutrient problem, a major pruning can all cause this to happen the next 
night. Also, if your aquarium has a high fish load or less than a "Dutch" 
amount of plants, it can happen regularly.

All I know is, the pH swing due to the loss of CO2 is minimal and gradual, 
and I've never had any fish stress or deaths due to pH swings. I *have* had 
fish stress and deaths due to O2 starvation, sometimes even when the plants 
were bubbling like
crazy the day before. I haven't lost a single fish in the 2-3 months I've 
been running the airstone at night.

If you're considering this, try watching your fish at 1:00 am for a few 
days. In my case, even when the plants were bubbling like airstones, the 
fish would all gather at the surface gasping for air a few hours after the 
lights went off. This usually didn't kill them, but it was stressful and I 
would occasionally lose a fish--and always a fast-moving fish that needed 
high oxygen levels.

I suspect this is mainly a problem that those of us with relatively high 
fish loads run into. I like to keep my one inch per gallon of fish, or a 
bit more, even in a planted tank. Amano's fish loads tend to be a bit high 
and some of his designs use a relatively small number of plants, so I'm not 
surprised that he does this too.

Injecting *pure* O2 is a novel idea; I imagine this could be done silently 
and, with a good diffuser, wouldn't drive off CO2. However, this would be 
expensive. More importantly, my aquaria are definitely NOT worth the risk 
of keeping a canister of pure O2 in the house...

- --
michael moncur   mgm@starlingtech.com   http://www.starlingtech.com/
"Let's have some new cliches."                -- Samuel Goldwyn


Zapping scum

by Dave Gomberg <gomberg/wcf.com>
Date: Sun, 05 Dec 1999

At 03:48 AM 12/5/1999 -0500,  TEACHSKIP@aol.com asked:
>Help, I have a white surface scum in my planted 55 gallon tank. I previously 
>got some advice that it was protein...  I have to strain this scum out every 
>day with a fine mesh net.   I withheld food and light and that did no good...
>Any suggestions???

I have reversed my opinion on use of airstones in planted tanks with
pressurized CO2 systems.  

Originally I thought that running an airstone from midnight to 6AM or so
was a waste of CO2 and that the pH drop that would occur in the dark if you
didn't run an airstone occurred in nature anyway and was of no importance.
 I still believe the latter, but....

Running an airstone during the very late night can remove (redistribute)
all kinds of surface junk.  Oily scums, algae (in my case), other trash.
I find it very useful in keeping the top clear.  All you need is a cheap
timer and a cheap air pump.  Less that $20 total.   

If you use pressurized CO2 the value of the lost CO2 that is driven off by
the aeration is next to nil.   If you use DIY (yeast) or chemical CO2, it
is better if you can store the CO2 produced during the night for the next
day.   But then that adds to the cost and complexity of equipment, maybe
you should just run pressurized CO2 anyway.  Your call.

HTH.    Dave
- --
Dave Gomberg, San Francisco            mailto:gomberg@wcf.com
My aquarium plant supply store:      http://www.wcf.com/store
For low cost CO2 systems that work:  http://www.wcf.com/co2iron 
Tropica MasterGrow in the USA:      http://www.wcf.com/tropica 
- -----------------------------------------------------------------


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