First start off with a 5 or 6 thesis statements and narrow it down from there. Then make sure you have enough material to back it up. And a plan on how you will do your project to collect the right data.
This is for High School, correct, not college? And you have a year? Your project with the tank needs to reflect what you are writing about so you will be collecting your own data?
Yeah genetics wise, I don't think I would use mollies. P. reticulatus are good because they possess genetics that have been researched in depth already. If you look at wild specimens from Trinidad (the original collect spot), all of the fish have different color patterns. This is interesting because it is the same genetic principles as the human eye. I don't know a lot about genetics, so it would be an interesting topic to me
Water chem will probably be good. Lots of meticulous record keeping. You also have a lot of research you will be able to find and use from other people like, pH and alkalinity relations, O2 and CO2 diffusion/dissolution rates, pH and carbonic acid relations. Oh and whatever you do, don't use the phrases "off gas" or "gas off." Use the terms diffuse and dissolve.
Wish I could have done a project like this in High School. I had to write a 20-25 page paper on a chosen literary subject my Junior year. I was assigned Le Morte d'Arthur. At least my Senior year I got to choose. So I took Mary Shelley's Frankenstein and the scientific philosophies of Dr. Frankenstein and his actions related to modern scientific philosophies. That was an easy A.