You can tell when your fish is dying or sick by............
- staying in one spot
- different personality
- keeps on floating up to the top then it moves a little and floats to the top again.
- the scales could be flaky
- white spots on the body (called ick) and let me tell you if your fish has it and you have other fish in the tank too they could get it. Ick is contagious to fish (only).
You can go to the pet store and get fish salt that can sometimes make the fish feel better.
I hope this helps and makes sence.