It's a lipidaceous substance that builds up on the surface not necessarily a protein although a skimmer, which really doesn't work too well in fresh water due to specific gravity, would remove this potentially. It's normal to get a film on the surface when you have no agitation up there. The blue drop checker tells us the CO2 dosing may be inconsistent, and resulting in wild pH swings in what potentially is very soft water (as others said give us some more params). I agree the fish do look a bit stunted-but not much you can do about that at this point. +1 to the salt bath, maybe move them to quarantine to get them healthy and get the tank figured out. Could be high ammonia + pH mayhem.