It sounds to me like ammonia poisoning. Cycling is the process of growing 2 colonies of bacteria in the filter, to process the ammonia produced by the fish, via nitrite, to nitrate. This takes weeks. Many local fish shops advise people to run the filter for a day or two, and call this cycling.
Given that you haven't given us any water statistics, I must assume that the filter is not cycled, and therefore, it is not processing the ammonia. I reckon that's what killed the glofish(R), and what is making the platy look poorly.
There is a link in my signature area to an article which details the best way forward for someone in your situation - it details how to grow those bacteria, without killing any more fish.