I have had the same thing a few times I think.....Is it when they are perfectly helathy one day, and then the next day their tails end up in a point? like they are glued together? I looked everywhere in books and on the net for a cure for this but couldn't find one. Ended up losing most of my guppy population......tried everything - added salt, malkolin blue, melafix, those little orange capsules with green powder inside....nothing worked. In the end I just emptied the tank down to the lowest possible and replaced nearly all the water - the already sick ones didn't make it, but prevented any more getting sick. The sad thing is that the males usually get it a few days after the females have died. Small water changes don't see to help the situation either.