this is related to flourish.... its happened to 2 times in the past as well...
what you want to do is a huge water change, and use stresscoat or something like that. its chemical burn on the fish....caused my an overdose or direct exposure of flourish on the wee fish...
don't mean to scare you.. but the first time this happened to me I lost 6 discus, and the second time i lost 900worth of fish including all my fancy plecos.... it was after that, i realised it was too much flourish.
don't medicate too much, but after the h20 change, add a tiny amount of meth blue and a tiny amount of furan in the water - this is to make sure they do not get disease since they have lost their slimy coating...
let me know how it goes... ive managed to rescue clown loaches with this so it is possible...