Generally, if fish like Platys blote up, it's a water quality issue.It can be on a couple of levels:
a) if you have soft tapwater, it's a stress on a fish like a platy, which is adapted to harder water. You might want to check with your local water company and see how hard or soft the water is;
b) more lilely, pollutants are building up, often because we love to (over) feed our fish. I'm surprised no one has told you to buy a test kit. It's the standard advice when we're unable to think of what to do - a do something response. I wouldn't - I would do a 30% water change every week if you have a lot of fish and every ten days if the stocking is light. Once a week is best.
c) it could be disease, but that comes around to the stress of water quality slipping.
This is all preventive stuff. For now, stop feeding after an immediate 40% change. Stay with that for a few days, and watch. When you restart, feed only a little. If it's a mild infection, it will pass. If it's a major, the fish might pass. Since it's very hard to know exactly what's wrong it's hard to medicate.
