I highly doubt that very tiny concentrations would harm fish, in fact I know it wouldn't, but the key is tiny. In SW tanks the dosage is something like 1 ml of vodka for a huge tank. SW fish and corals tend to be far more sensitive than most fw fish, and they do well and corals actually improve due to the subsequent nutrient depletion. So I guess the question is firstly, can plants use it, and secondly, is it of any use in doses that are small enough so as to be not harmful to fish? And perhaps thirdly, will it cause any unwanted effects such as massive overgrowth of bacteria which could harm fish secondhandedly due to oxygen depletion? Putting fish in a very concentrated vodka/water solution will kill them, it's one way to euthanize a fish. And yes, the fish will go upside down, but we're talking far, far higher levels than what people dose to SW tanks. But if it has the same effect as it does in SW tanks, that of causing a bacterial bloom instead of staying in solution for the plants, if indeed the plants can use it, it wouldn't be very useful. I'm not really sure how to measure the beneficial effects of something on plants, so I'm not sure if trying it is even worth it. Probably what I need are some fast growers and high light (to see effects quickly) and a control tank of course. I might be able to do this if I can come up with enough tanks. Really, three tanks would be nice (0, moderate dosing, higher dosing). Maybe even three 1 gallon containers would be enough to try something.