I think some people have claimed that some few month old media they have found in their attic or whatever still had some bacteria after being hydrated again. My Bio-Rocker becomes completely dry after my pump shuts off, and when it has restared, my fish are unaffected... but then I have serveral gallons of bio-balls, the denitrator block, and my canister w/ about 40 sq ft of area, so I can't really tell if the bacteria die. Actually when my pump is off, it stays off for a while, for several days sometimes(my pump sucks right now) since I'm too busy to reset the impeller. But when I do reset it, I keep it on for 24 hours.