My fish tend to get sick more frequently as I progress through the game.

Watch how often you feed your fish. Too much food makes them sick especially as now you have researched the fish food, you don't need to feed them so often. It's easy to forget and feed them anyway!

The fish medicine isn't expensive although it is irritating to keep having to cure your fish. For very sick fish (health in red area) boost their health with vitamins before feeding the fish in your tank otherwise your recently cured fish may die.