54 CO2 in the Aquarium
Description
This article is from the Aquaria: Plants FAQ, by multiple authors.
54 CO2 in the Aquarium
Georg Jander (GEORG.JANDER at cereon.com)
Anyone who has observed the explosive growth of aquarium plants in
response to carbon dioxide (CO2) fertilization must be convinced of
the usefulness of this system. Certainly, there are thousands of
aquarium hobbyists who do not give their plants any sort of special
treatment and still end up with a fairly nice display. However, truly
luxuriant growth, the sort that you see on the covers of aquarium
magazines and in pictures of "Dutch aquariums," can only be achieved
by fertilizing with CO2.
During photosynthesis, plants use light energy to capture CO2. This
CO2 is used to build the basic carbon structures from which all plant
material is made. In a poorly lit aquarium, light is likely to be what
limits the rate of plant growth. The amount of CO2 produced by fish-
and bacterial respiration is more than enough to allow photosynthesis
under these conditions. If on the other hand, you try to make your
plants grow faster by adding more light, it is likely that there will
not be enough CO2 in your aquarium. The plants simply can not grow as
fast as they would like to, given the available light energy.
The easiest way to increase the amount of CO2 in an aquarium is to buy
a tank of CO2 and let it bubble into the water. Several, mostly
German, companies sell systems for adding CO2 into the outflow of your
canister filter. If you buy your CO2 system from someone like Dupla,
you are likely to spend about $300. That seems a bit pricey, doesn't
it? Fortunately, it is very easy and also a fair bit cheaper to buy a
CO2 tank at a local welding supply place and use it to bubble CO2 into
the water.
CO2 in the tank is under high pressure. A pressure regulator brings
this pressure down to a manageable level, and ordinary aquarium air
valves can be used to regulate the flow to individual aquariums.
[Editor's note: this is counter to general net-experience. Most of us
end up installing a fine-metering needle valve after the normal
regulator in order to regulate the flow down to a few bubbles per
second, because normal aquarium air valves do not have good enough
control.] The CO2 reactor is simply a small chamber that allows the
CO2 to be dissolved in the water before it escapes into the air.
Outflow from a filter or a pump enters the top of the reactor; CO2 is
bubbled in from the bottom. To give the CO2 more time to dissolve, one
can add a system of baffles to trap the gas as it is moving up. Near
the top of the reactor, there should be a small hole to vent other
gases, which may be present in small amounts in the compressed CO2.
These gases do not dissolve as readily in water as CO2 does.
I purchased my CO2 tank and regulator at Wesco on Vassar Street in
Cambridge. Their current (May 1992) prices are: 5 lbs CO2, $52.50,
refill $9.74; 20 lbs CO2, $101.75, refill $19.55. A CO2 pressure
regulator is "$79 and change." People who have better welding
connections than I do might be able to get things more cheaply than
that. [Editor's note: look in the PLANT RESOURCES section for more
current prices and good inexpensive sources.] Refills are generally
not a very big expense. My 20 lb CO2 tank is used on three aquariums
(30, 65, and 110 gallons) and lasts about three years between refills.
That works out to about $2 per aquarium per year. Other possible
sources of CO2 that I have not investigated are CO2 fire extinguishers
and the CO2 canisters they use to put the bubbles in beer and soft
drinks. Don't bother trying to rig up something with dry ice, it is
too complicated.
The tubing and valves that I use for my CO2 setup are the sort that
one buys for use with the aquarium air pumps. It is better to get the
brass rather than the plastic valves, since it is easier to make fine
adjustments with them and they also tend to leak less. Even a tiny
leak can empty out a gas tank distressingly quickly. I check all of my
valves and connections with a soap solution and make sure that no
bubbles appear.
The CO2 reactor can easily be constructed out of any wide bore tube. I
use the lift tubes from an undergravel filter in my aquariums. Local
aquarium enthusiast Jim Bardwell does well with the top half of a
one-liter coke bottle, with the filter hose attached to where the cap
should be. It is best to use a clear plastic, so that one can see what
is happening inside. Baffles, designed to let the water cascade down
in one direction and to trap the CO2 moving in the other direction,
are helpful, but not absolutely necessary. I make my baffles out of
foam cubes that I cut to the right size and shape to fit inside the
tube. Jim simply lets the CO2 collect at the top of the reactor, where
the water is coming in. He does not have a vent and does not seem to
have a problem with excess gas accumulating.
While a small increase in the amount of CO2 in the water causes lush
plant growth, too much CO2 can prove to be toxic. CO2 dissolved in
water forms carbonic acid (H2CO3). With weakly buffered water, like
what comes out of the tap in the Boston area, adding too much CO2 can
bring the pH down to as low as 3. That is not quite as acidic as Coca
Cola, but about equal to vinegar. Naturally, this can cause death or
other serious reactions in your fish and plants.
One can buy CO2 test kits that measure the actual level of CO2 in the
water, but measuring the pH and counting the bubbles in the CO2
reactor works just about as well. It is best to start off by adding
CO2 very slowly (about one to three bubbles per minute) and increasing
the rate until a small, but measurable drop in pH is achieved. In my
30-gallon aquarium, I add one bubble of CO2 every three to four
seconds to bring the pH from 7 to between 6 and 6.5. How much CO2 one
needs to add varies from aquarium to aquarium and can depend on
several factors: the size of the aquarium, how fast the plants are
growing, the number of fish, how much food is decaying on the bottom,
the buffering capacity of the water, the types of rock and gravel, and
how well ventilated the surface of the water is. However, anything in
the range of one bubble every two to fifteen seconds seems to work
pretty well. Bubble size will vary with the diameter of the tubing. I
am referring to the sort of bubbles that come out of the end of
ordinary, one eighth inch inside diameter aquarium air tubing.
By using a CO2 reactor, you are saturating the water with CO2, and any
excessive agitation of the water surface or bubbling of air through
the water will cause the CO2 to escape into the atmosphere, just about
as quickly as you can add it. Thus, at least during the day, you
should *not* have an airstone or an undergravel filter turned on. If
you have a plant aquarium, you should probably not be using an
undergravel filter, anyway, since most kinds of plants do better
without one. When the lights are on, plants use CO2 and produce
oxygen. In my tanks, so much oxygen is being produced, that I can
often see it forming streams of bubbles from the plants. At night, on
the other hand, the plants are actually using oxygen (and not CO2) If
there are not too many fish in the aquarium, then the oxygen produced
by the plants during the day will tide everyone over until the next
morning. However, if you notice that your fish are gasping at the
surface in the mornings, they are obviously running out of oxygen. To
remedy this problem, you can simply turn on an air stone when the
lights go out. This will keep up the oxygen level and remove excess
CO2. I have the aquarium lights and an air pump on two separate
timers; when one turns on, the other one turns off. It would also be
fairly easy to rig up a solenoid valve for the CO2 supply and have it
turn the CO2 on and off with the same timer that is regulating the
lights.
The system that I have described here and use is a very basic one that
works well. For those who like those sorts of things, the automation
possibilities are almost limitless. My brother Albrecht, who is an
electronics whiz, has his entire aquarium run by a TRS-80 computer.
Among many other things, the computer measures the pH, adds more CO2
if the pH is above a predetermined level, and sounds an alarm if the
CO2 tank is running low. Fortunately, you don't need all of that to
have a truly great-looking plant tank. There are more than thirty
kinds of thriving plants in my aquariums; I have to weed out bunches
once a week, and I have enough extras to supply all of my aquarium
friends and still sell some at the monthly BAS auction. The fish are
also doing well and reproducing.
CO2 makes it easy to grow aquarium plants, but it is not a cure-all.
You still have to observe some of the other essentials of proper plant
care. Aquarium plants need a lot of light. When using fluorescent
bulbs, I usually figure about four watts per gallon. Wide-spectrum
plant and aquarium bulbs seem to work better than the "soft white"
ones that you can buy at the hardware store. The amount of iron in
most aquariums is too low for maximum plant growth. I supplement the
iron by adding "Micronized Iron" to the canister filter (about one
teaspoon at every cleaning) and "Ortho Greenol" directly to the water
(two drops per ten gallons per day). Both of these are available at
gardening stores. Other nutrients and trace elements that your plants
need are usually taken care of when you feed the fish and do water
changes (frequently). Also, don't forget the regular sacrifices of
goat entrails to the aquarium gods, at midnight when the moon is full.
Continue to:
My Books
