If your fish is hiding too much or is very skiddish, it is probably living in fear of something. Make sure no other fish are harassing it, and that there is not a large amount of movement around the tank. Adding plants will calm the fish. (even fake ones)
If your fish lines itself up with a rock, then swims at it real fast and scrapes on it, it is itching for some reason. This is not normal if done excessively. Possible causes are ammonia poisoning, a pH swing, or a parasite of some sort.
If your fish hangs at the top of the tank, it is probably having trouble breathing. Test for Nitrite and examine for symptoms of Gill Flukes.
If your fish seems to float or sink uncontrollably, it probably has Swim Bladder Disease (https://www.petfish.net/kb/entry/12/167/)
If your fish refuses to eat, it is generally a sign of stress. Look into you water quality and check for Signs of disease.
If your fish never moves much, check its temperature. Generally, the water should just feel a little cool to the touch for tropical fish. If it falls too low, it can cause a fish to slow down. Also look for other signs of disease.
For further diagnosis, see our Disease Articles Category https://www.petfish.net/kb/12/ to diagnose your disease. Also try entering a description of what you see into Google, “fish reddened gills” for example.