Not sure what's going on but figured it would be the older fish die offs. It seems to affect the older female guppies, starting off like "half tail" being white and all the way to their backbone to dorsal fin. I noticed instead of being opaque white on the upper part of body, inside looked white while the bottom looked dark grey. To the first sign to death, usually within 24 to 48 hours. They would get very sluggish, rapid breathing and their tails would get stiff acting like dolphins trying to get on a platform. I removed any dying ones and it is not affecting the youngsters YET and it has been four days. Just the older fishes are having problems. My barbs, tetras and ground dwellers and snails are not affected by this at all. The only thing I can say it may have been the new group of neon tetras and very young female guppies that I bought from the store, quarantined them in another tank, no ill effects but the "weakness" of my oldest guppy females didn't seem to weaken as time went on. But once I added the new young fishes, the die off starting to happen. I have a 55 gal tank so it is way plenty of room for guppies, guessing around ten to 15(leaving two older females, three very young males and the rest of them are just juvies), five tetras (two older, 3 very young), two barbs (very docile), one gold sucker fish, four snails, and two glass eyed fish (probably barbs or tetras). Right now it is on Melafix, first day yesterday and today, I didn't see any losses but the one older female has a total white color (still has her black lace pattern to it but whatever "clear" turned white )to her tail fin, her abdomen is tuning white (it looked like a white saddled GS dog pattern). Really it is not white on the scales but internally. I thought for sure it was fat underneath the skin but whatever it is, its spreading from topline down to her sides like an infection of some sort while her bottom line is dark grey. That is how prounounced the skin color would be instead of light grey to yellowish grey of normal females. What I would notice on the vivid colored tailed females, it would start off as white circular pattern on the tip of tail, starting on the top. I have continue with Melafix for seven days or unless someone tells me its the wrong treatment and need something else. So far in the last three days, I lost five older females some are very pregnant to gave birth prior to this outbreak. I certainly hope it will not attack the younger fishes which seems to be immune to this "disease" and I have two guppy males that I really want to keep. I lost their father about three weeks ago which he accidently got himself stuck in the filter part. he was one of those German blue mosaic color, just absolutely goreous. What is going on?