vodka is an organic carbon source. you can also dose sugar or vinegar. There's a lot of extensive write-ups on this subject. It is a controversial thing, however. Do a google search for "vodka dosing in aquarium" and you can find some really good essays. Some have had really good experiences with this, and some not. I really want to try this myself, but I don't want to screw up something that's already working fine. Let me know how it goes.