My strategy for in the beginning is to find the Golden Goldbulb, which typically sells for $60, and breed ten of them and then sell them after they give birth, but before they die on me. Because of this, I have probably sold over 1000 of these guys in total, but, of course, once I find something that will sell for more (after Fish Env. #1), then I'll sell of all of my GGs and breed the new species.
HTH
-spaceman