I have a Fluval 406 canister filter in my planted aquarium. Every time I clean it (every 3 weeks), the flow slows down significantly after about a week of being cleaned. When I first received my filter and set it up, it about shot water out of the aquarium because of its power. Now, even directly after cleaning, the flow is a lot slower, and worsens within a week. I have cleaned out my impeller several times and checked the tubing, yet I cant figure out what's wrong. I also hear some people clean out their filters every 3 or even 6 months. Should I not be cleaning mine so often? Anybody have similar issues?