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Co2 Systems
by George Booth
Not convinced about using CO2? Here's an e-mail I wrote trying to persuade someone of the benefits.
Question: CO2 injection scares me. I have always worked on the basis of a
balanced aquarium.
Think real hard about what it means to have a "balanced aquarium". I
assume you mean a type of natural equilibrium. You have 30 gallons of water and
a certain number of fish. You feed the fish. You provide some kind of filtration
to keep things from getting too foul. You change water regularly. There's not
really much "balance" here, because you must "manage" the
environment to keep things from getting out of hand.
Most importantly, the ratio of fish to water volume is incredibly high, compared
to the natural environment of a lake or stream. Considering the flow of water in
a stream or the natural turnover of water in a lake, you cannot possibly provide
the "water quality" fish would experience in the wild. Nature provides
a massive and complex system that keeps natural bodies of water
"balanced" that we aquarists cannot possibly duplicate.
Q: There is a system that plants create oxygen that is used by fish and
converted to CO2 which is again changed be the plants into more oxygen.
It is nice to think that the plants help the fish by providing oxygen and the
fish help the plants by providing CO2 and fertilizer. In most aquariums, this is
not true.
First of all, plants convert CO2 and nutrients into carbohydrates and oxygen
provided there is enough energy (light) to maintain photosynthesis. As plants
grow faster, they draw more CO2 and nitrogen and phosphorus from the water and
they generate more O2. This is good for the fish. However, fish won't produce
enough CO2 to raise the level above normal equilibrium levels (2-3 ppm) unless
you are massively overcrowding the fish. It will diffuse into the air as fast as
the fish produce it.
How do you tell how fast the plants are growing? Besides making physical
measurements, you can monitor the O2 in the water with test kits. You will find
that in a non-CO2 injected aquarium the O2 level is around 80-85% of saturation
(O2 saturation is 8.1 mg/l at sea level and 75 F, lower at higher altitudes and
temperatures). In a tank of water with no life and decent circulation, you will
find that the water is very close to O2 saturation due to diffusion at the water
surface. As you add fish and bacteria and other O2 users, you will find that
surface diffusion is not enough to keep up with the O2 demand, and the O2 level
drops to the 80-85% level. Even massive aeration (bubble wands, etc) is not
enough to get the levels above 90%. Even trickle filters will not raise O2 above
95%, contrary to the ads you see. This is not conjecture, but based on
measurements made in our home aquariums.
The only way to actually increase the O2 level of the water is to either inject
O2 with an O2 reactor (which no one does) or to get some biological process
going that can force additional O2, which is where plants come in. Keep in mind
that plants *only* generate O2 when they are photosynthesizing; at other times
(like at night), they are O2 users along with everything else. I have found that
without CO2 injection, the plants do not produce enough O2 to raise the O2 above
what a tank without plants would have. The only purpose they serve is to provide
decoration and hiding places for fish. They do not provide "balance"
in the biological sense.
With CO2 injection (and proper light and nutrients, of course), you can
physically see the plants generating O2. Bubbles form under the leaves and
streams of bubbles come from broken leaves and stems. We typically measure from
110 to 125% oxygen saturation (which is good for our fish, since we live at a
high altitude and they would see a deficiency otherwise). When plants are
growing this fast, they are also removing more pollutants from the water and
provide some of the balance you are trying to achieve.
Q: I do believe that by increasing the CO2 level in a tank the plants may
grow better (provided enough light, and nitrogen is also available) but doesn't
this also effect the fish in the tank?
Yes, this affects the fish in the tank by providing *more* O2 than they would
otherwise receive. Extra CO2 does not displace any oxygen, contrary to aquarium
myths.
Q: I remember many years ago reading an article on CO2 poisoning of fish in
a trade magazine and would like to hear more on your thoughts.
Fish aren't affected by CO2 levels until they get to the 40-50 ppm range. Most
people using CO2 injection maintain under 20 ppm. I would be wary of articles in
magazines and books - it's not difficult to prove any point you want to make by
citing the particular author that agrees with what you are saying. I would
suspect that most aquarists would agree that information from "many years
ago" has been found to be *very* outdated (and we don't agree on much :-).
Q: Is this CO2 injection system expensive?
$200 for a manual system * (less if you are a careful shopper and can find used
regulators and such). $600 + if you want an automatic system. I guess
you could call that expensive.
* Just Regulator, Diffuser, Check valve. Very bare bones; and difficult to control. Not what we use ourselves and thus, not what we recommend. You'll end up buying the other items (needle valve, reactor, Indicator kit, bubble counter) to make a complete Co2 system later and ultimately paying more.
+ Our Automated Delux Co2 system w/ controller is just $399. Very reliable and appropriate for planted aquaria.
~ Editor.
Q: I have always had great luck with amazon sword plants, and crypts, but
every time I tried Madagascar Lace Plants it seemed like I was just throwing out
money.
Amazon swords love CO2. Crypts don't seem all that excited about it. Lace plants
like colder temperatures, but do well with CO2 in a warmer tank.
Q: I originally started with 3 Amazons, and in less then 5 years I had at
least 6 Amazons in each of my 30 or more tanks, until I suckered into adding UGF.
One of our E. Bleheri produced 30 8" plants on a single flower stalk in 2
months, which was only one of a dozen stalks it sent up in a single year.
Q: If you have any articles that you can E-Mail me on this I will evaluate
them, and if they seem to have any merit I give it a try on one of my 30 gallon
set-ups.
Check out my photos in the "Photo Album". The tank with the
Rainbowfish has an UGF. All the photos are from tanks with CO2 injection. Also,
if you can find a copy of The Optimum Aquarium (Horst and Kipper, 1986), it goes
into much of the details of "the right way" to set up planted
aquariums (using the equipment their company sells, of course :-).
BTW, all this information is based on experience with tanks ranging from 85 to
100 gallons, but I suspect it will translate to smaller tanks. It might be more
difficult to maintain the proper CO2 levels in a smaller tank without a CO2
controller.
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From a posting by Pauli Hopea in Finland. The following table is from a Finnish aquaria magazine (Akvaariomaailma)
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
---------------------------------------------------------------------------
* [Easier to read Co2 /KH table]
~ Editor
Important note: KH is carbonate hardness. This is different than alkalinity. Use
a Tetra KH test kit to determine the KH. Although the Tetra kit most likely
measures alkalinity, it will be close if you do not have a lot of phosphates in
your water (from commercial pH buffering products, for ex maple).
Another important note: Watch out for measurement error. A difference in +/- 0.2
pH units and +/- 0.5 in KH can produce quite a range of CO2 values. Be aware of
your potential measurement errors!
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Here's a simple equation to calculate CO2 if you know pH and KH.
CO2 = 3.0 * KH * 10^(7.00 - pH)
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Want to increase your hardness?
One teaspoon (about 6 grams) of sodium bicarbonate (NaHCO3) per 50 liters of
water will increase KH by 4 degrees and will not increase general hardness. Two
teaspoons (about 4 grams) of calcium carbonate (CaCO3) per 50 liters of water
will increase both KH and GH by 4 degrees. Different proportions of each can be
used to get the correct KH/GH balance dictated by the fish and plants in the
tank. Since it is difficult to accurately measure small quantities of dry
chemicals at home, a test kit should be used to verify the actual KH and GH that
is achieved.
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Concerned about CO2 loss in a trickle filter? Don't be! (From an article posted
quite a while ago).
CO2 Loss in Large Aquariums
We are in the process of setting up a new 90 gallon plant tank based on concepts
presented in Dupla's "The Optimum Aquarium". Before we finalized the
set up, we had the chance to conduct some experiments in the bare tank. We ran a
"CO2 loss" test recently and wish to share the results with the AGA.
We verified that the major culprit in CO2 loss is surface turbulence (no
surprise here). However, counter to our expectations, trickle filters DO NOT
necessarily cause rapid CO2 loss.
The test tank was a "90 gallon" glass tank (48"x18"x24"
tall, 79 gallons of water). It was set up with Dupla heating coils and a Dupla
"DuplaTherm" temperature monitor/controller to maintain the water
temperature at 79.0 +/- 0.1 degrees F. A Dupla Reactor "S" was used to
inject CO2 via the sump of a trickle filter.
CO2 concentrations and pH were measured with LaMotte test kits. Note that the
LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an error of about +/-
2 ppm. The LaMotte pH test kit has a resolution of 0.1 pH units and uses an
"octet color comparator". The pH error is about 0.05 pH units based on
comparisons with a Sandpoint II pH/ORP controller and comparisons with a pH/KH/CO2
table.
An AquaClear 802 powerhead was used to circulate water in the tank. It was
placed near the bottom in three tests to provide a gentle circulation current
with little surface turbulence. A 0.3 ft/sec surface current was noted, giving a
smooth surface pattern that looked like "heat waves" rising off a
highway in the summer. In a forth test, the powerhead was placed at the surface
and was adjusted to give vigorous ripples without splashing.
The trickle filter used was an Amiracle "100 gallon" unit with a
bio-media capacity of 3.99 gallons. The media space is 16.125" long x
7.625" wide x 7.5" high. The media used was 238 Dupla BioKascade
bio-balls, with the internal slats arranged roughly horizontally to allow the
water to move through the media in a gentle, cascading manner.
The filtered water is circulated by a Quiet One pump controlled by ball valves
to provide a 400 gallon per hour flow, turning over the tank five times per
hour. The trickle filter has two water returns. One is directed across the
bottom 1/3 of the tank, providing a flow at what will eventually be the top of
the gravel. The other return utilizes a Magnum 330 water return fitting. To
provide surface turbulence, a Magnum diffuser was used to direct the return flow
across the surface, producing ripples equivalent to the powerhead when placed at
the surface. For tests without turbulence, the diffuser was removed, allowing
the water to be directed towards the bottom of the tank.
The tank is bare except for the equipment mentioned - no gravel, no livestock,
no plants. Lighting is room ambient. The top is open.
Before we turned on any equipment, we filled the tank with tap water and
adjusted the water hardness by adding 3 tablespoons of calcium carbonate (CaCO3)
and 1 tablespoon of sodium bicarbonate (NaHCO3) to achieve a GH of 3.5-4 degrees
and KH of 7 degrees as measured by a Tetra test kit. Note that GH has no bearing
on the CO2 measurements, but a KH of over 3.5 is needed to ensure accurate CO2
test kit readings. A KH of 7 was selected to keep pH readings in the range of
the LaMotte test kit.
After letting the water equilibrate for one day we measured dissolved CO2 at 2-3
ppm. We then set up a large powerhead to circulate the water (Project RS-500,
~500 gph) and let it run for a day. The CO2 remained about 2-3 ppm. At the end
of most of the tests, CO2 again measured about 2-3 ppm, indicating that this was
the equilibrium value for the experimental conditions (note that the altitude
was 5000 feet above sea level).
After the initial tests, the heating coils, trickle filter and CO2 injection
were set up. The large powerhead was replaced with the AquaClear 802. For the
first test ("trickle, turbulence"), the trickle filter was run with
the Magnum diffuser producing surface turbulence and with the powerhead running
at the bottom. For the second test ("powerhead, turbulence"), the
filter was turned off and the powerhead was placed near the surface. For the
third test ("trickle, quiet"), the trickle filter was run without the
diffuser and with the powerhead running at the bottom. For the last test ("powerhead,
quiet"), just the powerhead was used, running at the bottom. In all tests,
the trickle filter and reactor were used to get the CO2 level up to the point
were the test started. At that time, the CO2 was turned off and the reactor
allowed to clear of residual CO2 before actually starting the test.
Due to some difficulty in getting the CO2 to the same starting point for each of
the tests (actually, a lack of patience on our part), the CO2 readings were
normalized for the table below. To normalize the readings, the raw data was
plotted with the CO2 concentration on a log scale. A best-fit line was drawn by
eye through the data points. The numbers in the table below were then read from
the plotted lines at hourly intervals. Just CO2 data in the range of 10-33 ppm
is shown, since we consider that range to be the most relevant for planted
tanks. The raw data is shown at the end of this note.
CO2 concentration (ppm)
-----------------------
trickle, powerhead, trickle, powerhead,
Time (hrs) turbulence turbulence quiet quiet
-------------------------------------------------------------
0 33 33 33 33
1 21 24 28 28.5
2 13.5 17 24 25
3 - 12.5 20 21.5
4 - - 18 19
5 - - 14.5 16
6 - - 12.5 14
7 - - 10.5 12
8 - - - 10.5
At KH=7, the following table relates CO2 to pH:
CO2 (ppm) pH
-------------
42 6.7
33 6.8
26 6.9
21 7.0
17 7.1
13 7.2
10.5 7.3
8 7.4
What surprised us was the fact the trickle filter itself was not responsible for
much CO2 loss (compare the last two tests). It should be noted that air was not
pumped into the media chamber during the tests. We suspect that any out-gassing
of CO2 by the media will quickly produce a concentration of CO2 in the chamber
such that it reaches equilibrium with the CO2 in the water.
In our other trickle-filtered tank, we have noted very high CO2 loss (we go
through a 10 pound tank in 6-7 weeks). We now suspect that the loss is caused by
the Eheim canister filter spray-bar return. We plan to run further experiments
on that tank to verify this conjecture.
Although some authorities recommend pumping air into the media chamber of a
trickle filter, we have found no evidence of a need for this. Thriving plants
will provide plenty of oxygen for the aerobic bacteria colonies during the day
and we have noticed no problems at night when the plants are at rest. We ran a
long term test on another tank using a Sandpoint II pH/ORP controller and found
no difference in ORP with the air pump on or off. We also noted that less CO2
was used with the air pump off (longer intervals between CO2 bottle refills).
From the table, it would appear that a 10 pound CO2 tank will last about 5
months when we finally get the new tank set up. We will have a KH of about 5 and
will regulate the pH to be 6.8 +/- 0.05. This is a CO2 concentration swing of 5
mg/l (27 mg/l to 22 mg/l) times 300 liters and should occur within 1.25 hours
for a usage of 29 grams per day. Of course, the usage by the plants will
increase this by some amount, but that's another experiment!
CO2 test raw data
-----------------
CO2 concentration in ppm and measured pH ()
Clock trickle, powerhead, trickle, powerhead,
Time turbulence turbulence quiet quiet
-------------------------------------------------------------
6:00 pm - - 23 (7.0) 47 (6.65)
7:00 pm - - 20 37 (6.75)
7:30 pm - 33 (6.8) - -
8:00 pm 27 (6.9) 27 (6.9) 17 31 (6.8)
8:30 pm 20 (7.0) 24 (7.0) - -
9:00 pm 17 (7.1) 19 (7.0) 14 27 (6.9)
10:00 pm 11 (7.3) 14 (7.2) 10 23 (7.0)
11:00 pm 8 (7.4) 11 (7.3) 10 21 (7.0)
12:00 am - - 9 17 (7.1)
3:00 am - - - 15
8:00 am 2 2 - 8 (7.4)
12:00 pm - - - 5
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Curious how the LaMotte CO2 test works? Here's a description from their water
testing handbook.
CARBON DIOXIDE FOR WATER AND SEAWATER
Introduction
Carbon dioxide is present in all surface waters, generally in amounts less than
10 mg/L. However, higher concentrations are not uncommon in ground waters.
Dissolved carbon dioxide has no harmful physiological effects on humans and is
used to recarbonate water during the final stages of water softening processes
and to carbonate soft drinks. High concentrations of carbon dioxide are
corrosive and have been known to kill fish.
The analysis for carbon dioxide is similar to that for acidity. A water sampler
is titrated to a phenolphthalein end point with Sodium Hydroxide Standard
Solution. Strong mineral acids are assumed to be absent or to be negligible in
effect. Care must be taken during the analysis to minimize the loss of carbon
dioxide from the water sample as a result of aeration when collecting and
swirling the sample.
Chemical Reactions
The reaction of sodium hydroxide with carbon dioxide (as carbonic acid) occurs
essentially in two steps, first a reaction from carbonic acid to bicarbonate and
then to carbonate:
CO2 + H2O -> H2CO3
H2CO3 + NaOH -> NaHCO3 + H2O
NaHCO3 + NaOH -> Na2CO3 + H2O
Because the conversion of carbon dioxide to bicarbonate is complete at pH 8.3,
phenolphthalein can be used as a color indicator for the titration. The sodium
hydroxide titrant must be of high quality and free from sodium carbonate.
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