I'd like to start out by saying I enjoy your Sand Mail column each month, as I am an advocator of the plenum system as well. Have a few questions regarding this system, so I'll get right to the point. I've heard controversy over the use of the plenum system, and one argument in particular concerns me. I've heard the extended use of a plenum can cause build up of nutrients under the sand bed, which leads to nutrients leaking out into the system. I'm not sure what to think about this, as I've only heard good reports from friends, and strangers alike using this method of filtration. It seems plausible; yet at the same time, a functioning plenum system should break down these nutrients, not accumulate them.
In regards to inhabitants of the sand bed, I've noticed you do not recommend the use of any benthic inhabitants that might stir up the sand bed. This I don't understand. I've been taught it's essential to stir up the sand bed weather using this natural system or not, to enable it to breath. It seems logical that a layer of sand will prevent itself from clogging and fouling, therefore, if something is not in or on top of it, mix it up.
If you know of any drawbacks to using the plenum method, to naturally filter water, please let me know, as the nutrient sink comment was the first I'd heard of. Thank you once again for the time you spend on your column, as it is most informative!
Thanks for the nice words. For those that say a plenum system crashes after a year or two, they won't say that directly to me because I can prove them wrong! There's not been a plenum crash, not one, where the cause was the method itself! Those that have not faired well were found to be either deficient in their original construction or on-going husbandry.
As for nutrients building up in the plenum (space under grid), that is somewhat true, yet that is one of the purposes of the plenum - to take nutrients out of the loop and hold them as an energy reserve for the bacteria on the sand above, which in turn causes those levels to vary. A very dynamic feature of plenum systems! There is no truth in them leaching back into the aquarium. As for leaching nutrients back into the bulk water, the deep sandbed 'directly' on the aquarium bottom is problematic because nutrients MUST buildup at its bottom area because its electrical charge becomes more negative with depth and keeps on attracting nutrients. That's not the case with plenum systems as its bottom/plenum electrical charge is slightly less negative than the sandbed above, which now returns those nutrients/feeds the bacterium on the sand particles above.
As for stirring the sandbed, the whole concept with plenum sandbeds is to maintain anoxic conditions in most of the bed, not aerobic (by stirring) or anaerobic. Keep in mind aerobic conditions result in nitrate, anoxic conditions result in the nitrate being broken down into its elemental forms. Of course, anaerobic areas, where there is no oxygen, are to be avoided as nitrate is only reduced to ammonium and no further, which benefits unwanted algae.
That's why plenum systems are so much better than anything else out there because their volume of anoxic areas is far, far greater than areas in the common DSB system! Think about that for a while - it's only common sense when you really think about it!
Don't confuse stirring with vacuuming, as shallow vacuuming, nothing deeper than 2 inches, is recommended once a month. This also checks the bed for any areas that may tend to harden/clump from fast forming bacteria growths or calcium carbonate precipitation, which can then be broken up with the fingers if found. This 'shallow' disturbance will not unduly upset the plenum processes, as those in this area will quickly, within a day or two, reestablish themselves. Hope this helps.