I've had my blue platys dropping like flies. They are the offspring of other blue platys in my tank. Oddly enough, the parents are doing fine. It's only the 3-month old fry that are dying. None of the other fish in the tank are showing any bad signs at all -- thankfully.
I've been led
to believe that it's a product of too much inbreeding. Seems that lots of people are having lots of platy problems over the past few months. The hatcheries are being blamed by many. I'm still a rookie at this hobby, so I don't really know. I'm just repeating what I've read/heard.