I got lucky with one of my early games. I was just trying to cross-breed like crazy while I tried for "stable" fishes as I was researching the advanced fish envirnoments.

I got lucky because I got a pair or orange snoopers. They weren't very stable, but the pair of them kept them alive longer than ususual for a fragile breed (they died as soon as I fed them, but before that I recorded what I needed to recreate them.) I bred more pairs (the Magic Fish of Fertility is your friend...) and kept a pair in each tank.

I then was able to breed the canary fire-arrow while still in Level 2 envirnoment, and then I was able to quickly earn enough to buy the Level 3 research.

Later games I just bred until I got the golden goldbulb and then set of a breeding farm for that to earn money. (Yes, I red the rumors of a better fish to breed... I haven't found any besides that fish of fertility...)