Hey guys, so the flow on my HOB filter (marina s15) slows down every week. I've found that cleaning the impeller every week gets the flow back up to normal for 1-2 days before it slows down again. When I take out the impeller to clean it, it usually isn't very dirty. This has been happening for a few months now. What do you think the problem is? Should I get a replacement impeller, or is this an issue with the motor? Thanks!