cheers, all
I haven't stepped on the plane yet...
Indeed, yes... increased surface area has its advantages, but we need to be crystal clear here on our (limited) expectations for a restricted DSB like this:
It will become biologically active on a microscopic level of course... but not much more than that.
This really is about reducing nitrates... and not much more.
The lack of "food" in the form of solid matter is the very thing that prevents such deep, fast flow beds from becoming nutrient sinks. Yet we can still enjoy (or cannot stop) the possible denitrifying faculties from pulling from the constant stream of dissolved matter in the water always above the bed.
That pet shop 55gall DSB noticed nitrates dropping in less than a month.
As far as the question of "how does diffusive movement occur"... I would ask the following question in reply: how does it not occur? In a rather wacky analogy... do consider if at the bottom of your DSB bucket, barrel or tank... you first poured gasoline enough to fill the vessel 1/10th. Then you fill the rest of the vessel with dry or live sand... and turn it all online. Would anyone like to venture a guess if the gasoline at the bottom of that vessel will ever make it to the surface, or diffuse otherwise to the extent that it influences water quality?
I think it will
The DSB is not so literally sealed off from or isolate from the system water. It is fluid - literally... actually, with the delightfully unstable nature (round) of oolitic grains of sand in the case of aragonite.
The application summarily has limited benefits, limited risks, limited expense... and limited baggage (long-term). Its just a deep bucket of sand
If you have to buy bucket and sand new... the bucket will cost you more
Worst case scenario... it doesn't work, and you wasted $10. It won't be the first $10 you waste on your reef tank. heehee
But I have honestly seen this simply application work many times. For a normal (not over)bio-load... it can make a difference.
Let's find someone here online that has consistently struggled with some reasonable (readable on a test kit) levels of nitrate, to just try it... take readings at the beginning of the month, than again after one month... after a second month or more if they are willing. And let us know some details about that particular case. Bucket size, sand amount, system size and bioload, feeding/fish/coral load... etc.
kindly,
Anthony