I use emp. 400's on all my tanks. I think they are great filters. My question is how long can the bb on the bio-wheels remain healthy while not running.(ex. when completely dry, damp, or shortly after filter is turned off). When I do water changes some take quite a while, I leave the filter running till it starts gurgling then it is turned off till the tank is almost full. (usually a hour+, the bio wheels are still damp when the filter is turned back on). Anything that's going to take a long time I will take the bio-wheels out and put them in a bucket of tank water.