hi and welcome! A platy is too large for a 5 gal
unfortunately. I'm not sure if there isn't enough room in the filter for the bacteria to grow to sustain a platy since a 5 gal
filter is pretty small. That's my best guess but I could be wrong.
Are you using dechlorinator with water changes? Does it say it detoxifies chlorine, chloramines and heavy metals? What water are you using? What test kit? What's the temp in the tank? More info would help pinpoint a potential issue other than what I mentioned above.