CO
2 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 (CO
2) 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 CO
2.
During photosynthesis, plants use light energy to capture CO
2. This CO
2 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 CO
2 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 CO
2 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 CO
2 in an aquarium is to buy a tank of CO
2 and let it bubble into the water. Several, mostly German, companies sell systems for adding CO
2 into the outflow of your
canister filter. If you buy your CO
2 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 CO
2 tank at a local welding supply place and use it to bubble CO
2 into the water.
CO
2 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 CO
2 reactor is simply a small chamber that allows the CO
2 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; CO
2 is bubbled in from the bottom. To give the CO
2 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 CO
2. These gases do not dissolve as readily in water as CO
2 does.
I purchased my CO
2 tank and regulator at Wesco on Vassar Street in Cambridge. Their current (May 1992) prices are: 5 lbs CO
2, $52.50, refill $9.74; 20 lbs CO
2, $101.75, refill $19.55. A CO
2 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 CO
2 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 CO
2 that I have not investigated are CO
2 fire extinguishers and the CO
2 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 CO
2 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 CO
2 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 CO
2 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 CO
2 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 CO
2 in the water causes lush plant growth, too much CO
2 can prove to be toxic. CO
2 dissolved in water forms carbonic acid (H
2CO
3). With weakly buffered water, like what comes out of the tap in the Boston area, adding too much CO
2 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 CO
2 test kits that measure the actual level of CO
2 in the water, but measuring the pH and counting the bubbles in the CO
2 reactor works just about as well. It is best to start off by adding CO
2 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 CO
2 every three to four seconds to bring the pH from 7 to between 6 and 6.5. How much CO
2 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 CO
2 reactor, you are saturating the water with CO
2, and any excessive agitation of the water surface or bubbling of air through the water will cause the CO
2 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 CO
2 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 CO
2) 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 CO
2. 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 CO
2 supply and have it turn the CO
2 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 CO
2 if the pH is above a predetermined level, and sounds an alarm if the CO
2 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. CO
2 makes it easy to grow aquarium plants, but it is not a cure-all.
180G REEF/1-400W MH/4-140w 60 INCH ACTINIC VHO/2-37w 36' 10k t-5's/(1,034 total watts ...60G duel BERLIN BAG SUMP SYSTEM/MAG24/RO-DI SYSTEM/duel tds meter/PINPOINT PH/Euro reef rs- 180 skimmer.
