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CO2 ``FAQ''


  1. CO2 injection; many questions
    by (George Booth) (6 Mar 1995)

CO2 injection; many questions

by (George Booth)
Date: 6 Mar 1995
Newsgroup: alt.aquaria,rec.aquaria

Joseph Chong ( wrote:
> I was wondering if anybody can provide me with information or places where I 
> can find info on construction of a DIY CO2 injection system. I went the store 
> and saw the TETRA sytem today, on 'sale' for 399.00 CN$. The canister is the 
> size of a bottle of mousse!!! There must be a better way. Thank you in 
> advance. Please e-mail me at

From the personal FAQ of G.Booth: 

QUESTION: How can I get my plants to really grow well?

Plants, like all living things, need the proper nutrients to thrive.
Unlike animals which can hunt for food, plants need a proper
environment to supply them with the basics needed for survival and
growth.  The mechanism that drives plant grwoth is photosynthesis and
photosynthesis requires light for energy and CO2 to drive the chemical
reactions.  Various other elements are needed during photosynthesis to
create the carbohydrates which are stored and later used for growth.

The process of photosynthesis requires a specific light energy
threshold.  If the light is not bright enough, photosynthesis will not
occur.  Beyond that threshold and up to some high light level,
photosynthesis will run faster and faster.  Depending on aquarium 
depth and the types of plants, 2 to 3 watts per gallon is normally 

It is important to note that if any one of the chemicals required for
photosynthesis is in short supply, that shortage will be the limiting
factor on the photosynthesis rate.  Typical aquariums are deficient in
both CO2 and iron (Fe) with respect to optimum levels.  Various "trace
element" solutions are available to the hobbyist to meet iron and other
trace element requirements and CO2 injection can be used to boost the 
concentration of CO2 in the aquarium. 

QUESTION: Isn't CO2 harmful to fish?

In high concentrations, CO2 can block the respiration of CO2 from the
fishes gills and cause oxygen starvation.  Since the gills depend on a
CO2 concentration differential between the levels in the blood and the
water to transfer gases, high levels in the water will reduce the
amount of CO2 that can be transfered.  Although different references
have wildly varying values for toxic levels, a concentration of below
30 ppm is definitely safe.  Plants do best at around 15 ppm of
dissovled CO2.

It is a common misconception that water can hold only so much
dissolved gas and adding CO2 will displace oxygen.  This is not true.
As a matter of fact, if enough CO2 and light is present to enable
vigorous photosynthesis, oxygen levels can reach 120% of saturation.

Even at night, when the plants stop using CO2 and start using oxygen,
the oxygen levels wil stay about the same as a typical non-planted

QUESTION: What's the deal with water hardness and why is my pH 
so high?

In some regions of the country, the water supply is in close contact 
with limestone.  These regions are said to have "hard water". 

There are two things at work here.  Limestone is mostly calcium
carbonate (CaCO3), which slowly dissolves in water and dissociates
into Ca++ and CO3--.  The calcium ions (Ca++) contribute to (or are
the main component of) general hardness, abbreviated GH.  GH does NOT
affect the pH but is the type of hardness that biologically affects
organisms.  If something prefers hard or soft water, it is GH which is

The carbonate ions (CO3--) are a component of alkalinity, also known
as acid buffering or carbonate hardness (KH).  Actually, KH measures
carbonate and bicarbonate ions and is the predominant component of
alkalinity.  Anyway, KH is a prime determinant of pH.  High KH means
high pH.  The carbonate buffering system is the "natural" buffer for

GH and KH are can be measured in "degrees" or "parts per million
(ppm)".  Neither measure is very useful in scientific calculations,
but most test kits use one or the other so hobbyists generally use
them (degrees are more prominent).  One degree KH (dKH) or GH (dGH) is
equivalent to 17.8 ppm of CaCO3.

QUESTION: Is there a way to reduce pH without resorting to algae
inducing phosphate buffers?

Inject moderate levels of CO2 to the aquarium. 

QUESTION: How does one control pH with CO2?  

In most cases, the aquarist balances KH (which raises pH) with CO2
(which lowers pH).  For example, we are blessed with soft tap water,
both in terms of KH and GH.  We add sodium bicarbonate (baking soda,
NaHCO3) to get 4 degrees of carbonate hardness (4 dKH) as measured
with a Tetra KH test kit (cheap and effective).  With a typical
equilibrium dissolved CO2 level of 2-3 ppm, this gives us a "natural"
pH of around 7.7.  We then inject CO2 to lower the pH to 6.9, giving
us a dissolved CO2 level of 15 ppm, which is just about perfect.

We use electronic controllers to maintain a stable pH.  The controller
measures the pH.  If it is too high, the controller turns on the CO2 
to lower it.  When the pH gets below the set point, the CO2 is turned
off.  Since CO2 will try to equilibrate with the atmosphere, it will 
slowly diffuse out of the water (and be used by plants), again raising
the pH.  So the pH is, in our case, slowly going back and forth from 
6.85 to 6.95.  In another tank with a cheaper controller, it goes
from 6.80 to 7.00.  

In a tank with no controller, we have a constant slow flow of CO2 that
just balances the diffusion rate and plant usage and maintains 7.0 +/-
0.2 (higher in the daytime when plants are using CO2, lower at night
when plants and fish respire CO2).  These pH swings have caused no
problems with fish ranging from discus to angels to rainbowfish to
small tetras.

QUESTION: What equipment is needed for CO2 injection?

As far as we are concerned, none of the systems offered by the
aquarium industry people are that cost effective.  Various bits and
pieces are good, but no one system combines high quality, hgh capacity
and low price.  If you have access to the right suppliers, you can
assemble a much more capable system for less money.

Here is what is recommended:

Gas Bottle

Get a 5 pound to 20 pound CO2 tank from a welding supply shop.
Welders use CO2 to provide an inert atmosphere so they should be easy
to find.  If not, check for compressed gas dealers.  Other sources
include fire extinguisher suppliers and beverage dealers.  Careful
shopping may uncover a used tank that will be more cost effective.  We
pay $70 for a new 5 pound tank and $120 for a new 20 pound tank.

The place where you buy the tank should be able to refill it.  Some
will refill while you wait, some will trade a full bottle for an
empty, some will send it somewhere to get filled.  Naturally, you
don't want it disconnected from the aquarium for very long or your pH
will shoot up.  We have an old 2.5 pound bottle we connect up while we
take the regular bottle to be filled.

In our area, a refill for a tank 20 pounds or smaller is the same
price ($10), so the bigger tank saves money in the long run.  A 10
pound lasts more than twice as long as a 5 pound, and a 20 pound lasts
more than twice as long as a 10 pound (they have trouble getting all
the pounds into the tank, so 0.5 pound out of 5 is a higher percentage
than out of 10, etc).  


The best is a two-stage regulator designed for use on welding gas
cylinders.  This reduces the 950 psi tank pressure to 10-20 psi.  You
can get a cheap fixed-pressure regulator with no gauges for about $45
from places that have beer brewing supplies.  The lowest cost is a
Flow Regulated Orifice Gauge, FROG, and puts out a constant pressure
(22 psi or so).  These are cheap but there are no gauges so you don't
know when the bottle is about empty.  Without gauges, you'll just be
surprised some day when you check the controller and find your pH is

We paid $70 for an adjustable regulator with high and low pressure
gauges.  The high pressure gauge tells you when the bottle is about
empty.  The bottle pressure will stay at 950 psi as long as there is
liguid CO2 in the bottle.  Once the liguid is gone, the pressure will
begin to drop (2-4 weeks to go from 950 psi to 200 psi).  When the
pressure is around 200 psi, you should recharge the bottle since the
regulator gets a little flakey at that point.  We have a small 2.5
pound bottle that came with a starter set for use when the main bottle
is being recharged.


For an automated setup with a controller, a solenoid is needed to shut
of the CO2 flow.  We found a commercial unit for about $60.  It's
definitely over-kill but it has never broken.  It's good for 200 psi
and it's non-corrosive.  If you have a local commercial plumbing
supply store, they might have them.  Also look up "valves" in the
phone book.

The solenoid goes after the regulator on a CO2 cylinder, i.e., the 
low pressure side.  Some solenoids use the inlet pressure to help 
keep them off and may not work in this application.  

The regulator usually has a 3/8" NPT thread and the solenoid may have 
a 1/4" or 1/8" NPT thread, so you will need to find a place that 
sells up and down thread adaptors.  Most good hardware stores and 
plumbing supply places have these. 

Fine Control Valve

After the regulator, you will need a fine control valve to get the
extremely slow flow rate you need (1 bubble per second for a manual
setup).  Typical aquarium needle valves won't work - you will find
they are either all on or all off in this application.  A decent valve
will run from $10 to $40 depending on what you find.  Check the Yellow
Pages under "valves".

We have one by NuPro (model B-4MG2) that cost $35 and is superb.  We
have another from The ARO Corporation, phone (419) 636-4242, called
the NO1 for about $10, but it's obsolete now and is a little harder to
get adjusted just right, especially in a "manual" system.

Bubble Counter

To help set the flow you would need a "bubble counter" or some way to
see the actual flow.  If you inject directly into the tank or you can
see how much is going into the reactor, this is optional.  We like the
Dupla bubble counter ($35) because it also includes a check valve.
You don't want water to get into regulator (water + CO2 = carbonic
acid which is mildly corrosive)!  Aquarium air line check valves may
work, but the one I once used had an internal melt down when presented
with carbonic acid).

A reactor of some sort is needed.  We use Dupla reactors in our
trickle filter sumps but almost anything that will allow the CO2 and
water to mix will work.  One of our tanks has the CO2 bubbling into
the intake of an Eheim canister filter.  Works fine but is not as
efficient as a purpose-built reactor.  The best thing is to work out a
"counter current" thing where water flows from top to bottom in a
large tube and CO2 bubbles up against the water flow.  You may be able
to adapt a cheap protein skimmer for this purpose. 

A manual setup works fine but takes constant fiddling to balance it.
The CO2 bubbles in continuously (DON'T shut it off at night) and you
have to balance the inflow against plant usage and diffusion into the
air.  You will get pH fluctuations of about .3 over the course of a

An automatic system is great if you can afford it.  A controller costs
around $250-300 and the solenoid is another $60.  Electrodes must be
cleaned every few months and replaced every 18 months or so ($90). 
But a controller will keep the pH stable at +/- 0.05 units.  
A CO2 test kit is handy to have but optional.  The Lamotte kit is the
best we've seen, about $25. You can determine CO2 levels from accurate
measurements of pH and KH.  Lamotte makes a great narrow range pH test
(6.5 to 7.5) that you can interpolate to 0.05 pH units. The Tetra KH
kit seems about the best for that and is dirt cheap.  We also have a
Lamotte alkalinity kit (KH) but prefer the Tetra kit.

If you need to get a "system", I would recommend a Sandpoint system
over a Dupla system.  It's a bunch cheaper.  We bought a regulator +
solenoid + fine control valve combo from them a couple of years ago
that has worked OK but the fine control valve was worthless.  Perhaps
they've improved that.  They also offer a reactor.  

We like the Dupla reactor "S" and the bubble counter.  Their gauges,
regulators, solenoids and valves seem very overpriced.  Also, be
careful that stuff you buy is compatible with the CO2 bottles offered
in your country.  German fittings don't work with American bottles,
for example.

QUESTION: How is the final flow adjustment done?

The CO2 regulator should be set to about 20 psi on the output.
Typical regulators get a little flakey at pressures below that.  The
fine control valve should be adjusted to give you about one 1/8"
bubble per second (the rate that is best for your specifc tank will
vary depending on all kinds of factors).

QUESTION: What are "CO2 reactors" and why are they needed?

There are all kinds of ways to get the CO2 into the water.  Some are
very efficient (and may need a controller to avoid wild fluctuations)
and some are less efficient, wasting CO2 but providing a margin of
safety.  The basic idea is to get the CO2 bubbles in the water and
keep them in contact with the water as long as possible.  

The simplest is to bubble CO2 from a airstone on the bottom and have
the CO2 bubbles come up below a powerhead or filter outlet so they get
pushed around the aquarium.  Get a good airstone so the bubbles are as
fine as possible.

The CO2 can also be directed into the intake of a canister filter,
giving it plenty of time to mix before it comes out the outlet. 

There are three or four commercial reactors available; check the mail
order ads in FAMA. 

QUESTION: Do you have any experience with Sandpoint's pH controller?  

More than we should have ...

We have two Sandpoint Dual pH & ORP controllers, one Accurate I pH
controller (the new style) and one ??? (old style) pH controller.  Be
sure you get either the Dual (very "overkill" and $480) or the
Accurate I ($250).  DO NOT get the old style (black case).  The
controller usually comes with a pH electrode and calibration

Solenoid valves are not included with the controller. You can either
get one from Sandpoint for $90 (nice unit but over-priced) or scrounge
around local plumbing places trying to find one ($20 to 70,
depending).  Probably the Sandpoint unit is worth the extra money just
to avoid the aggravation of trying to tell the salespeople what you
are trying to do.

QUESTION: Do you think that if a plant tank's pH is routinely
stailized between 7-7.2, that there will be sufficent CO2 present to
meet the needs of the plants?

It depends on the amount of carbonate hardness (KH) present.  KH, pH and
CO2 have a fixed relationship as long as carbonate is the only buffer
present (no phosphate buffers like pH-UP and- DOWN, Discus Buffer,
etc).  You can determine CO2 concentration by measuring pH and KH
and using the table below.  An optimum level of CO2 is 15 ppm; a 
good range is 10-20 ppm.  Keep in mind that good CO2 levels also 
require good light levels and proper nutrients and trace elements
to be effective.  All things must be in proportion. 

The El Cheapo Tetra KH test kit is as good as any and is actually
more useful than more expensive "alkalinity" tests kits for this 
purpose.  A pH test kit with a resolution of 0.2 units is also 
suggested.  Pay close attention to the accuracy - if the KH kit is 
accurate to +/- 0.5 dKH and the pH kit is +/- 0.2 units, the range
of CO2 values is quite large.  Let's say you measure 3 dKH +/- 0.5 
and the pH is 7.0 +/- 0.2.  This would indicate CO2 ranging from

    KH = 2.5, pH = 7.2  -> CO2 =  5 ppm 
    KH = 3.0  pH = 7.0  -> CO2 =  9 ppm
    KH = 3.5  pH = 6.8  -> CO2 = 16 ppm

which could be good or bad!

The following table is from a Finnish aquaria magazine
(Akvaariomaailma) and was posted by Pauli Hopea.

The relationship of CO2 , pH and KH
 \ pH | 6.0     6.2     6.4     6.6     6.8    7.0    7.2    7.4    8.0
KH\   |
0.5   | 15      9.3     5.9     3.7     2.4    1.5    0.9    0.6    0.2
1.0   | 30      19      12      7       5      3      1.9    1.2    0.3
1.5   | 44      28      18      11      7      4      2.8    1.8    0.4
2.0   | 59      37      24      15      9      6      4      2.4    0.6
2.5   | 73      46      30      19      12     7      5      3      0.7
3.0   | 87      56      35      22      14     9      6      4      0.9
3.5   | 103     65      41      26      16     10     7      4      1.0
4.0   | 118     75      47      30      19     12     6      5      1.2
5.0   | 147     93      59      37      23     15     9      6      1.5
6.0   | 177     112     71      45      28     18     11     7      1.8
8.0   | 240     149     94      59      37     24     15     9      2.4
10    | 300     186     118     74      47     30     19     12     3
15    | 440     280     176     111     70     44     28     18     4
      |                 CO2  milligrams/liter

George Booth                         "Nothing in the world is more dangerous             than sincere ignorance and conscientious 
Freshwater Plant Tank Technology     stupidity" - Martin Luther King, Jr. 

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