Check for cracks in your o-ring. I have used Hagen fluvals on over a dozen tanks over the past 15 years and totally love them. Though I would recommend using two of the 300 models and not mess with the 400 series. I have never owned a 400 series because everyone I know who has had one had problems. I have used the 100, 200, and 300 series with no problems for a long, long time.