In my 50 gallon, I have been having some deaths a platy died, a guppy died and now a rummy nosed tetra is dying. I noticed that behind the gills there is a pinkish redish coloring (looks rather irritating), is this gill disease? if not, what could it be? I need to find out so I can treat the problem before this disease kills all my fish. BTW, I don't see this abnormal coloring on all the fish in this tank, just some of them. Thanks in advance!