In our house we have started a couple decent size fish tanks. One is a 55 gallon, the other a 26 gallon. In the 55 gallon we are grown a decent amount of aquatic plants and do not have a lot of fish.

To compensate for this we actually bubble CO2 into the tank 8 hours a day so that the plants will have enough CO2 to grow rapidly and clean the water of excess nitrogen from the fish waste.

The nitrogen cycle in a fish tank
In a fish tank the fish release waste in the form of ammonia. Then certain nitrogen fixing bacteria change the ammonia to Nitrite and a different nitrogen fixing bacteria change the nitrite to nitrate. Nitrate can then be consumed by the plants, although weekly water changes are necessary to remove excess nitrate.

Edit: The above note about the Nitrogen changing from Ammonia to Nitrite to Nitrate may or may not be True under all conditions? Yet, the problem of Excess Nitrogen waste as a whole may be the problem. End Edit.

In a fish tank, the problem is excess nitrogen not CO2
Whenever we see someone having problems in their fish tank it is related to excess nitrogen. Many times people will not include live plants and place too many fish in too small an aquarium. This can create excess levels of all three forms of nitrogen (ammonia, nitrite, and nitrate). Fish are particularly sensitive to ammonia and nitrite especially if the pH of the water is higher than 7.0. Experienced fish keepers try to keep their water pH between 6.8 and 7.0.

This gives me an interesting experiential perspective on the concept of global warming. The excess nitrogen from misguided agricultural practices is a much greater danger to the environment than CO2 emissions. Moreless has been saying this all along.

Keeping a fish tank has really afforded me a place to see the pH balancing principles in practice. The water in the aquarium is like our lymphatic system. Keep in balanced with all the right minerals and your fish thrive. Overwhelm it with too much nitrogen and you begin having problems.