I have a 20gal saltwater aquarium with 2 false percula clowns, one pink mushroom rock, one green mushroom rock (which isnt doing so well), one purple mushroom rock, a blue and green clam, a diamond watchman, and a koko worm. For about a month now I've had this brown film over the surface of my water. I've done 2 water changes and it goes away but then it comes back. What is this stuff? It separates when I add food to the water like oil or something. I dont have a protein skimmer but I have opticarb which is supposed to act just like it. I change it once a month. Anyone know what this 'film' could be from??