thats probably ich... most likely due to stress from your high nitrates. i wouldnt do anything too harsh since clown loaches have "skin" maybe the heat treatment. i wouldnt separate him. maybe treat the whole tank since ich is contagious.
i replied on your other thread... but since you cant use salt (or shouldnt) i say turn the temp to about 30-31c. makes the ichs metabolism speed up and they detach from the body and the heat kills them.
If they run short of O² they'll congregate at the surface and possibly gulp air.If your filter outlet moves the surface around enough you can get by without an airstone and pump.It wouldn't be a bad thing to have though and they are pretty cheap to buy.
