The way to earn more money early in the game is to breed fish that will sell for at least $50 (there are several that will survive to at least age 50 in your starting environment) and keep breeding those fish over and over until you can get to Level 3 of Fish Environment. You will have some failures as you experiment with cross-breeding, and a lot of the babies will probably die until you get to a better fish environment, but the payoff is worth it. Playing it safe and breeding only common fish will allow you to make money, but it will take a lot longer, since most of them sell for very low prices.