When you're just starting out, it's not a good idea to cross-breed your fish indiscriminately, since you'll create a lot of fragile hybrids that will die before they reach selling age. Fragile hybrids are not suited to the Level 1 Fish Environment that you have at the start of the game. It's better to breed fish that will actually live long enough to sell them, so that you can make money.

There are a lot of successful strategies for getting started. You can find a few of them here.