I have Whisper Power 30 HOB filters on my tanks. There is a floss filter you can put stuff in, and a foam filter that sits behind it (closest to where the water flows back in the tank). I rarely rinse out the foam. I rinse the first filter in tank water during water changes, and then put it back in. When it is at the falling apart stage, I replace it with new and change out the carbon. What I've been doing is when I completely replace the filter, I take the old one and stuff it in the filter housing to help reseed the bacteria, but I never know how long to keep it there. Does anyone else do this? How long does it take to re-seed a new Bio-Bag?