How do you keep a fish healthy?

llness in any animal is not difficult to spot, and in fish any which are not feeding as normal must be watched carefully for further signs of regression.  Any fish that seems to be having difficulty swimming, or is swimming at an angle, is obviously ailing. 
Look carefully at skin and fins.  If there are any little white spots on them the fish probably has an illness called fish fungus (it will look like bits of cotton wool).
Those with bulging eyes, compared to their normal state, and those which are rubbing against stones or have growths of any kind on their scales are further examples of unhealthy goldfish and should be netted with care and placed in an isolation tank containing only water. In such an environment the fish is free of stress, and medication appropriate to the symptoms can be given without affecting the healthy members of the fish community.   You can take your fish to a vet in a small, plastic tank and sometimes people in a pet shop may be able to help by testing the water in the tank and providing some basic medication.