DIY Co2 Injection for plant tanks                    by Jessica Bullock, H&DAS

Aquarium plants require three basic things: light, fertilizer and carbon dioxide (CO2). In order to achieve optimal growth, a balance of the three is required; too much of one and not enough of another will result in poor growth. When light levels exceed two watts per gallon, supplementary CO2 is required for most aquariums. This can be achieved by CO2 injection. There are commercial set-ups available, but a much cheaper alternative (especially if you're just experimenting) is do it yourself CO2 injection.

The two basic components of a CO2 system are the generator (where the CO2 is made) and the reactor (where the CO2 is dissolved into the water). The generator is a 2L pop bottle, connected to some airline tubing. Simply poke or drill a hole through the cap, stick the tubing through it and seal it with the aquarium silicone sealer.

The CO2 will be generated by a yeast culture. This consists of sugar, yeast and water. Using ¾ cup sugar, 6 cups of warm water and roughly 1 teaspoon of yeast will give you the steadiest supply of yeast for the longest time. The culture will die off because as the yeast consumes the sugar, it produces CO2 as well as alcohol, which builds up to levels that are fatal for the yeast. When you mix up the culture, allow it to cool before you install it into your aquarium system; you don't want to siphon you tank water into the culture (a check valve is a good idea!). One batch will last approximately ten to fourteen days (one bubble every 3-7 seconds); you will need to replace your culture after this amount of time. To avoid large fluctuations in the amount of CO2 that you're providing your plants, try to have a new batch of yeast culture ready a day or so before the old one expires.

For a reactor, you can simply run the airline to the bottom of your aquarium and allow the CO2 to bubble to the surface, dissolving as the bubbles contact the surface. If the flow from your generator is strong enough, you can use an air stone to make the bubbles finer. Providing a site where the CO2 can slowly collect and dissolve is a much more effective and efficient use of your CO2. Running the airline under an upside-down dish or bowl in your aquarium lets the CO2 stay underwater for longer, allowing more of it to dissolve into your water. The airline could also be run into a power filter or powerhead, where the bubbles can be forcibly broken up and dissolved. If you try this method, be sure to test it out before actually applying it in an aquarium, you don't want to suck your yeast mixture into your aquarium.

Injecting CO2 into your aquarium will change the chemistry of your water, and make sure you monitor all conditions carefully when you first install your unit. The CO2 will form carbonic acid when it dissolves, which will lower the pH of your aquarium. Be very wary of pH crashes if your carbonate hardness is less than four degrees; water this soft does not have the necessary buffering capacity to prevent crashes.