The most likely cause of the declining health of your fish is that your breeding strategy is producing fragile hybrids that can't survive in the Level 1 Fish Environment that you have at the beginning of the game. Some hybrid fish that you produce can survive until at least adulthood in a Level 1 Fish Environment, at which point you can sell them, and some hybrids will die before they're grown. It's always safe to breed
identical common fish together (the fish that you started the game with: Sharks, Betas, Spotanus, Fatfish, Fruitfish, and Stickfish). There are several fish that will live to at least age 50 in a Level 1 Fish Environment and sell for more than $30 (there are some that do much better than that). The trick is to discover fish that you can sell for a better price, without producing fish that won't live.
While you're experimenting, though, it's important to take care of your original fish, or keep enough money for replacement eggs, in case all of your hybrids die. There are a number of recent discussions in the forum that discuss various strategies for early success in the game.