I would sug...buyin a test kit and checkin the water, also what do you have in your tank. One think with aquaclears is, it's recommended to discont...the pump housing from the unit and clean it out every 3 months.
I've got 3 oscars in my tank right now, and the aquaclear 300 and 500 keep the tank crystal clear. keep us posted so we can better help you out... Oh yea, what all do you have in the tank, other then the obivis...fish speices... Drift wood, plants... so on and on and on ...