At the beginning of the game, before you've researched higher levels of Fish Environment, you need to avoid breeding different species of fish. Doing so will create fragile hybrids, which can live only in better tank environments. Try to breed identical or nearly identical fish and sell them until you have enough money to improve your research level. There are some hybrid fish that sell for better prices and will live long enough in your original tank environment to breed and sell them, but you'll probably have a lot of dead babies during your search for them.