Hi I keep having a problem with my guppy’s tails losing color and the guppy dying. The tail seems more or less intact but has no color. Maybe it has lost membrane (or whatever). Doesn’t seem nipped or frayed. I do have tetras in the tank. (Photo is of guppy in its own tank now). This is the 3rd guppy this has happened to. Overnight. The first one was a hardy guppy that didn’t seem to have any problems. Lived with a few tetra for a long time. This one in the photo I just got yesterday. Along with the tetra. They had been in the same tank at the fish shop. I can’t promise it is a good fish seller. Also they swim at the top of the tank. Any advice? Knowledge? Thank you (can you see the photo? Not sure how to attach. I uploaded it as an attachment.)