The fish that die because of the tank environment don't exhibit overt symptoms. They just lose health and die. Only the "common" fish will completely thrive in the initial tank environment. There are some "hybrid" fish that will survive long enough to reach adulthood and sell. You may even have some survive long enough to breed them, but be prepared to have a bunch of fish die before you discover the ones that will live. Several of the hybrids will sell for more than $20 and will help build cash faster. You're doing exactly the right thing early in the game - breed fish that will live and manage your money carefully!