I used to keep platys in my 29-gallon. When I first started keeping fish, the pH in that tank ran between 7.2 and 7.4. I don't know what the hardness was, but I suspect that the water was at least a medium hardness. Anyway, I slowly transformed that tank to one for fish that prefer soft, acidic water. I had one male platy that "withered" away. The rest of my platys were fine. Eventually, they died one by one -- from old age, I guess, but maybe not. All I know is that after the tank became a soft, acidic water tank, if I put another platy in there to keep the last couple of survivors company, it would "wither" away and die. Soooo, my guess would be that you should test your water's pH and hardness. Your pH should be between 7 and 8 and your hardness should probably be in the 12 - 25 neighborhood.