Seems like the co2 output from a batch of beer might be very high for a very brief period of time, and therefore not very helpful. You don't want to gas your fish, or have the co2 levels fluctuate so much.
However, you certainly can add co2 to an aquarium by fermentation. Google diy co2, or search threads on a forum like Plantedtank.net.
Before I went pressurized, I used diy co2 on my 29 gallon and my 20 gallon, and it worked quite well. I still use diy co2 on my 2.5 gallon nano tanks.