I maintain a 200L tank at my work. One of the black Mollies had been biting at the scales and eyes of the Pearl Gourami. The other 4 mollies do not bother the PG, and none of the mollies bother a dwarf gourami in the same tank. Over the weekend, the PG has lost most of the scales on its head, and part of one eye. I isolated it from the rest of the tank as soon as I saw how much damage the M was doing. Any suggestions? Is this common? Thanks.