So over the past two weeks, a sort of oily scum has formed on the surface of the water. When it breaks up it doesn't go back as it's like rather solid. The other night I removed all (or nearly all) of it by placing paper towels on the surface. However, today it's returned after 3 days. Its so solid that it keeps my CO2 bubbles underneath it. Its not green, it doesn't really have a "colour" Any ideas? Pointing the filter outlet up more to cause more ripple doesn't solve it.