http://theaquariumwiki.com/HeximitaMostly due to poor water quality, malnutrition, stress etc.
If you take the fish to the vet (or know someone) you should be able to get metronidazole tablets with dose rate.
Or try to source some clout tablets, i have a couple from a member on here and TF awhile ago.
if anyone is interested PM me and i'll give you his username.
I had a pair of Afrcan shellies with it a few years ago, started out like small pin holes around the forehead then turned to bigger holes. Both fish died.