My Fish is sick How do I make it better on Fish Tycoon

Well, when you start Fish Tycoon, it's best to breed only the same type of fish, because then they'll be more likely to survive and be healthy.

Also, you can go to supplies and buy medicine. These are some illnesses:
ick looks like white spots on your fish
fungus looks green/brown

Generally, though, its not worth it buying medicine if you don't have much money.