You both are referring to PH and bacteria and some sort of substrate here. This thread discussion is about cleaning the substrate / sand bed in particular.
Bacteria are PH sensitive --- bacteria action will lower the surrounding PH --- striations are formed in a sand bed from bacteria action / water flow / light intensity / and wave length, etc,.
Clean the sand bed with a siphon and look at it tomorrow --- striations are present and they are in band and come all the way to the top of the substrate in areas. There is not only aerobic bacteria right below the surface there is anaerobic bacteria there. Either way in the area just below the surface there is a low PH environment that reduces the size of the sand particle in time. There is gas exchange, and mineral exchange to a very small degree --- as the particle / crystal of aragonite lets say is quickly stopped from any further chemical action "further growing or completely dissolving into the water" by Magnesium or Phosphate.
Adding calcium or the snow storm effect ---> results in the action of crystal poisioning. And, has been covered by authors like Randy Holmes Farly and others.
Phosphate --->
http://www3.interscience.wiley.com/journal/119905306/abstract
The deeper the bed the less overall affect the sand bed has on water mineral chemistry.
However, if the bed is allowed to remain undisturbed, the more waste is accumulated and the more the PH of the surrounding environment is lowered. Yet, the water flow over the surface of the substrate reacts very little as this deep action is inches below the water surface. Your tank PH is lowered.
Chemists know this and still recommend a cheap form of Calcium and Carbonate like Kalkwasser. Why ---> because it has a higher change of achieving the desired effect.
IMHO keep the substrate "of any kind" clean to reduce VOC's and Nitrates.
OFM
