Kingfisherfleshy,
Re Isopropyl, I don't have any first hand knowledge, but note a couple things. Its more toxic than ethanol, and can be a pain to handle. In humans it is not nearly as easy to metabolize as ethanol. While the fact that we can't use it as easily as we "use" ethanol does not necessarily mean the bacteria would face the same problem, it does lead me to believe you can't know for sure it will actually work as a carbon source. Coupled with its toxicity, I just would hesitate to dose it. Personally, I would not use it in an aquarium, though keep in mind this is all theory.
Ethyl alcohol is what you find in Vodka, if no impurities are present that would be fine, so it depends on what source you are getting it from. For example, denatured ethanol has benzene added to get rid of water, and I would not add benzene to an aquarium.
re dilution, that is largely pointless, that's what happens when you add it to the aquarium anyway, or your doing it wrong. :sad: