interesting, i also have a slightly discernable odor emitting from the tank when i open the hood. i also have bubbles on the top of my tank, what causes these bubbles? i am currently using a 250 biowheel filter with two filter media, one was recently changed but the other remained to keep some of the bacteria. this may be a strange question but is this smell uncomfortable for the fish? my tank is not cloudy, just minor debris floating around.