A deep gravel bed requires a plenum to function correctly. Without a plenum to seal in the reactions the DGB does not function to eliminate nitrate. The plenum should be a fine sand and should be separated from the gravel by a fine nylon screen. This system is potentially dangerous in a freshwater tank since you don't have the sand sifting organisms like serpent stars, sea cucumbers and narcissus snails that you'd use in a reef tank. Without these organisms you will get toxic buildup of carbon monoxide, methane and sulfur dioxide. The sand sifters break up these pockets and release the gasses in very tiny amounts before they can build up to levels that would kill your fish. Manually stirring your plenum is not a good idea for two reasons. 1.) It's a 24/7 job. 2.) If you push the siphon or whatever you are using to stir to deep and allow water from the tank into a level below the nylon screen you will upset the chemistry going on below and stop the denitration process and not even know it until stuff starts dying in your tank. (It takes much longer to begin the denitration process below the plenum than it does to cycle a tank so if you do attempt to try this in a freshwater system do regular water changes for the first four months before reducing your water changes. Also, really MKF is not the best place for freshwater plenums. Big fish in crowded tanks don't work with DGBs & plenums and your big fish will eat any freshwater sand sifters IF you can even find some species that will do that job and not breed excessively)
