Serpae tetra has red patch near gills and pale body?

Ok so my 32G wasn't good anymore so now i have to wait until sunday to get a new tank running. I had 4 serpae tetras in a bucket for 48hrs. Today i transferred them to a 5G until i get a new tank. They all seem fine except for 1. He is pale and has a red patch near his gills. The others 3 had the color come back to them and seem fine. Could ammonia poisoning only affect 1 fish even if the rest were in the same conditions for the same amount of time?