There are a lot of helpful suggestions in
this thread. The most important thing to remember is that you need to keep at least a few fish that will thrive in your initial tank environments, or you will end up with no fish and no money. Cross-breeding early in the game almost always produces fish that can't survive for very long in a Level 1 Fish Environment. If you sell all of your initial fishies, you won't have any breeding stock that you can use to make more fishies if all of your hybrid babies die.