I have a relatively new tropical tank (Jan 2009) and I have had trouble with what looks like mold ("fluffy" white growth) on my filter. It grows on the top, moist areas of my filter and inserts. What could be going on and what should I do? It has a cover of sorts on it and I've wondered if it might help if I don't put that on so that it's not an incubator for the warm, moist conditions. What do you think?