Alan that was my theory that the bacteria survives a lot longer than people imagine. It is possible that those who are saying that when they turned it back on lost fish maybe there entire system was not properly kept in the first place. My friend will have his off at least week and turn it back on with no problems. Two weeks ago I had a spill from one of my canisters so I turned of my Eheim 2217, and Fluval 405 because I did not know which filter caused the problem. Turns out it was the Fluval. I forgot to turn back on the Eheim. This unit was off almost two weeks before I realized it. I plugged it back in and though it smelled it did not cause any problem and I have some of the most sensitive plecos out there.