Removing protein film from surface
This is usually from flake foods or decaying organic matter, you can as others have mentioned syphon it out or soak it up with some paper towels. I usually position my filter returns so that they cause some movement at the surface, just enough to keep any of these organic films from accumulating. Black mollies are also quite good at consuming this film. If your filter return is breaking the surface of the water you should not have to worry about gas exchanges.
PS. Is this a saltwater tank? I ask as skimmers don’t really work well in freshwater, plus even with a skimmer running on a saltwater tank, such films do develop and surface agitation is usually the best way to keep them from accumulating.
Hope that helps Giancarlo Podio, LMD
Category: Philippines Internet Koi Society
