That is why I ask because my Eheim just keeps working. I let it go 3 months once after reading that other Eheim users wait that long but boy when I cleaned it then I really was dirty. I am an ex-reefer and I guess I am still concerned about that crud still being in the tanks eco-system.JanS said:I have never really noticed a slow in the water flow in the Eheims even after 6 months, so I just use the above schedule with good results.