Get the right mix, breed the canary fire-arrow like crazy, and you can sell them for $475 all day long! I haven't been able to get even a dollar more than that. Would like to hear from someone who has, though.
There are two good fish that you can get early in the game that can survive in the starting fish environment. One sells for at least $100, and the other sells for at least $54. I'm sure others have found even better fish than these! You can use these fish to quickly get to Fish Environment 3, and then start breeding the expensive fishies.