Cool idea Travis and this could tie in well with many different scientific fields. i don't think you have to limit yourself into thinking that you have to come up with some sort of far left field ideas either. Some times its a great idea to focus on some thing that is commonly known and try to show why it is, or perhaps that it isn't actually true at all. Show good scientific practice, good observation, and that you tried your best.
Some ideas
Improving water quality -showing how a filter works -make your own model, show possible improvements...
Explain behaviour of fish (Pavlov) -use fish to prove they have a longer memory than commonly perceived
Evaluate how cleaver different types of fish are at cracking some sort of puzzle to have access to their food, use the same trap on different types of fish to observe if any of them are smarter than others
Conduct your own investigation of the genetics of a type of fish (guppies tails) to show how common traits emerge through bloodlines, or hypothesis has to from where of off shoot is if they produce any throwbacks
Good idea to display and photograph your fish ideas extensively, take many photos through out the process, you can always delete any you dont need after the project.