A strong possibility is that you started breeding Magic Fish before you purchased Level 3 of Fish Environment. There is only one Magic Fish that will survive for long in the initial fish environment. Most hybrids will start to lose health at various rates in Level 1 Fish Environment until they finally die, and the more fragile hybrids will die before they reach age 6 in a Level 1 Fish Environment.

None of the Magic Fish are invincible. They will starve to death if you forget to feed them, and sickness can kill them, as well. Neither the Magic Fish of Health nor the Magic Fish of Curing will guarantee that you'll never have a sick fish, or that you don't have to cure fish that do become sick. They just reduce the incidence of disease and make it easier to heal fish that get sick.