I know you are an extremely busy individual, but I was wondering if you could comment on your views of the slow plenum siphoning technique that some aquarists are utilizing and/or recommending after the plenum has been in extended service. Personally, I don't think this is needed and would probably only cause setbacks in plenum operation, (even if done slowly) but wanted to compare my thoughts on this with someone who would definitely know better than I. Anyway, thanks for your time.
I've seen this question asked many times. Some aquarists, especially those who speculated the plenum system was something similar to a septic tank where nutrients checked in but didn't check out, claimed the plenum area should periodically be drained to prevent build-up of nutrients. Unfortunately they spread the word that accumulation of nutrients in the plenum would soon leach out into the aquariums bulk water and kill everything and/or cause a major algae problem. Little did they realize only some accumulation of nutrients occur because that is another purpose of the plenum - to take nutrients out of the loop until they can be processed by the very efficient microbial processes in the above sandbed. An energy reserve so to speak. So a nitrate reading of something above what is found in the bulk water is a very normal happening and far from anything worrisome. Just part of Mother Natures control over existing, sometimes shifting parameters. Furthermore, it has never been found that these slightly accumulated nutrients leach into the bulk water.
As for siphoning the plenum, its possible it could bring too much oxygen into the plenum area and its sandbed changing anoxic areas to aerobic and thereby changing the electrical charge value of the bed and plenum area. This alone could cause a new chain reaction of events and effects, limiting equilibrium. There were also some aquarists who claimed to be using their siphon system to test different plenum water parameters. Unfortunately they appeared to be unfamiliar with the necessary protocols for testing to achieve accurate results. Without monitoring redox and dissolved oxygen parameters throughout the plenum and sandbed for a baseline, test results must be considered quite questionable. And as for a very slow withdrawal of water, its simply a wasted effort as it serves no useful purpose.
For those who have not spent six years of their life researching sandbed parameters in all kinds of situations both in the wild and closed systems, its easy to come up with logical sounding ideas. However, after six years of research it became evident why plenum systems performed as they did! Once the more involved microbial pathways were understood, and especially the importance of the associated electrical charge at different levels of the plenum and its bed of sand where unprocessed nutrients in the plenum area are attracted back to the above sandbed for reduction, siphoning was no longer logical or should even be considered a wise recommendation!
Hope this helps,