Only the common fish will remain healthy - those are the fish that you can get from common eggs. If you breed only those species of fish, they will live. Common fish are the Beta, Fatfish, Fruitfish, Shark, Spotanus, and Stickfish bodies with Greenfin, Orange, Pink, Silky, Spined, and Twin-Fin fin types. Those combinations won't lose health and die in a Level 1 Fish Environment. Good luck!