Agree with ppo. They are stressed and/or ill. I've had platies that looked like that, and they sometimes didn't recover. I don't have much luck with platies, period, for some reason. Yes, tell us what your PH, ammonia, nitrites, and nitrates are. Also like ppo said, sadly, fish purchased from a Petsmart or Petco more often than not tend to be sick, or are incubating something.