I turn the unit off, close both valves, when u take them off the lid have a rag cuz water will drip a little, siphon a couple gallons out of the canister into a bucket so its easier to carry, pour your bio media in the bucket, take the canister, lid, trays and foam outside and hose the hell out of everything! Just shake the bio in the bucket, place back in trays, zip it all up and put valves back on, turn both open and fire up!
I like to take my motor housing off and clean out the impeller just to be safe, but its not necessary, do this before you place baskets and media back in.
That's how i do it.
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Go S.Vettel #1 RB8 3X WDC!!!!!!!!!
http://www.monsterfishkeepers.com/forums/showthread.php?504763-Cheap-plants-less-nitrate!-POTHOS