Observe the fish; take in to mind what it normally looks and acts like then compare that to what it is acting like now. Check the fish for signs of illness:
- Lack of appetite
- fuzzy white patches
- white spots
- bloated appearance
- red spots or streaks
- deteriorating fins
- discolorations
- kinked back
- mushy abdomen
- red belly
- cloudy eyes
- "pop" eye
- clamped fins
- bottom sitting
- scratching/frantic
- lethargic
- swim bladder problems
- gasping at surface
- pale gills
- ulcers
- mouth deteriorating
- dropsy
- stool changes etc.
Once you know what the symptoms of a sick fish are then you can begin a course of treatment.