I've heard its because your skimmer is on a new tank? This skimmer is bout a yr old I took it off my 50g (became a reptile enclosure)and put it on my 15g . I know its coming from the skimmer. I tried sponges, foam ,everything and it won't go away!!!!can I just use lots of chaeto? Its a small tank with 2clowns and going to put in some hardy corals (zenya ,polyps ,leather coral,mushrooms) with a 5g water change a week ?? That's last resort ,please help!!!!!