The results of using Mutation Liquid are "random in a predictable sort of way."

It is random which (or how many) fish it will affect when you drop it in the tank, as well as whether it will affect the fin or the body of each fish that is mutated. However, there are only two possibilities for each mutation. For example, if the body of a Fruitfish is changed, it will become either a Fire-Arrow or a Flashfish (and nothing else). To see exactly what the possibilities are, you can do a lot of mutations and track them, or you can refer to some of the spoiler charts that other have already created, which are in the Fish Tycoon Guide section.