(This is more likely the possibility if you see your fish jerking (also called flashing) or rubbing its sides on any tank ornaments (reported more than once, fish have been seen doing "flips," but really they are scratching their sides on the bottom surface of the tank. Sometimes, the stress is visibly obvious, as these little parasites attach themselves on to the fish and somehow dis-attach the scales from the body).
I hope I helped you
Whatever it was, my fish are fine now. I would suggest you just keep an eye on him and see if it gets worse.
And watch for that other fish too.