I would check all of the above. But I would also ask if you are running one of those push on filters? If you are take if off and see if the bubbles go away. I had put one on my powerhead only to find tons of bubbles filling up my tank. When I took it off the bubbles went away, Oh and I found the filter was cloged up with tons of gook and alge. I now run it with just the plastic and no filter inside. I originaly put it on to keep my inhabitance safe anyway not for extra filtration.