I would first do water parameter tests to see if you have elevated levels of ammonia, nitrite or nitrate; that your water hardness is correct, that your ph level is all good and same goes for temperature. If these are all fine, then the next thing I'd ask is, is the fish new? Have you introduced any new fish or plants into your tank, or possibly ornaments, any of which might be carrying a disease? New fish are often stressed from moving home and being in a new environment, so the behaviour you've mentioned is typical for new fish. You can help your fish by dimming the lights, not approaching the tank tpp often (only to feed them once a day), and by adding in some stress coat. If the fish is not new and there is no other reason why it shouldn't be stressed (it could be stressed from bullying etc), this is typical behaviour of a parasitic infection. It can be remedied with a mutlcure to cure parasitic infections, and some aquarium salt at a rate of one level tablespoon per 20L of water (dissolve in some warm water before adding to your tank), and add in some stress coat.
