Every now and again a fish will, for no apparent reason, virtually waste away; turn out to be hollow bellied, listless, and lie around on rocks or on the sandy bottom in a most dejected manner. The contributing factors to this malady are underfeeding, an unsatisfactory diet, overcrowding, or water which has been too warm or too cold over a period.
Fish suffering from wasting seldom survive. I have tried numerous treatments with out any one single case of real success. The only treatment to try is a general conditioning. Permit lots of room for movement, feed live food (beef scraped with a knife is also generally an acceptable tit-bit), chopped earthworm or daphnia.
Fish do eventually die of old age, in which case they naturally waste away.