There is this excelent thread on another board that I frequent that is very intresting. The discussion is on DSB and their care and what they can do for us and what they cant. Its very informative and you all should read it.
http://www.reeffrontiers.com/forums/...ead.php?t=9544
Heres the first post of the thread by Anthony:
With a conversation shared with Mike (Mojo) last week in my mind, a post on RC from another interesting chap (Melev) prompted me to update my rant...er, advice/opinions on using substrates in aquaria.
I have posted this in mutliple places, and know that our friends here at RF are all about the information/sharing (and won't get uptight like the big BB boys will/do.).
FWIW, my (first) pass on substrates/sand-stirring, etc:
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... we can start a discussion of "to stir or not to stir" regarding substrates.
I say "substrates" because we can/should really include all grains and depths in this discussion.
Reason: all substrates (grain/depth) have the potential to become nutrient sinks in time.
in fact, because water flow is so poorly understood, incorrectly applied, and usually inadequate in aquaria... many substrates Do(!) indeed become nutrient sinks in time.
This is not the fault of the media (sand/gravel)... but rather the application(!) (typically overstocked fish tanks, overfed, inadequate water flow, poor skimming, weak water changes, etc.)
Yet that still doesn't change the fact that in real world applications (albeit commonly misapplied), substrates generally do accumulate excessive organics in time.
So why, you may ask, are there some DSB folks (myself included) that go years and years with no trouble at all? That's not luck, but rather the fact that they tend mostly to be the really good aquarists that are diligent to avoid the above mentioned pitfalls. You will find more often that not they stock fish lightly, feed the tank lightly, and/or are often good about (larger) water changes, and most importantly... have exceptional water flow.
Having the correct type of water flow is THE most crucial key to succeeding with any type of substrate. Solids must be kept in suspension long(er) for adequate opportunities for filter-feeders to utilize the, skimmers to export them, mechanical filters to trap them, etc.
If instead you let that, say... teaspoon, solid matter (dish waste, excess food, algae in the tank, detritus, etc) each day find its way slowly to the bottom of the aquarium, then you can imagine what a teaspoon of solids per day, every day, month after month, etc can lead too.
Dissolved organic levels increase, and there is simply a lot of solid-ish unwanted matter suffocating that bed, and worse... leaching undesirable elements into the water.
A neglected substrate of ANY depth is a huge burden on water quality.
So then you may wonder... well, what of all that life in the sand bed? The infuana, meiofauna, whatever-fauna? Aren't they growing, flourishing... recycling or somehow utilizing those nutrients?
Horse puckey!
Now wait... don't get me wrong. I AM a fan of DSBs (although I never recommend them to newbies or clueless folks).
And... I do believe that there can be desirable organisms in the living substrates that we keep/culture.
BUT... in the real world, our substrates (shallow or DSB) are frankly, deserts.
Most everyone stocks their displays too early and with the wrong kind of fishes. A majority of popular fishes kept in reef tanks will decimate the macroscopic life forms that we believe are or should be growing in our sand beds.
This is a benefit that I never subscribed too on DSBs. Its a joke. The biodiversity of an aquarium sand bed is weak at best.
Now having said that... I still find it useful to run living substrates.
but come on... lets be realistic about what a living substrate can do.
You get some microscopic activity. With a little effort, you can get some good natural nitrate reduction. And some tanks do indeed grow a few wonderful macroscopic organisms... various worms, some gastropods/snails, limpets... indeed some microcrustaceans.
But to speak of "biodiversity" to any appreciable extent in aquarium substrates is a weak argument.
I like living substrates (again, principally oolitic DSBs) just for aesthetic, buffering support of water chemistry, and very easy/efficient nitrate control.
in turn, I am aggressive about nutrient export (skimming, water changes) and have extraordinary water flow that hopes to even begin to come close to what we see on a reef.
As for flow with sand and issues/concerns of sand-storms/milky tanks etc.... again:
Horse-puckey!
The argument against using fine substrates because the water flow needed for animal health will disturb the matter is flawed. Ignorant at best, or just a cheap shot from the DSB-haters
Its a simply matter of having the wrong kid of flow. Too many folks limit their effluents to just a few ports. So for big aquariums (say 100-200 gallons), to have 2-4 Tunze streamers, OM's, SQWDs, etc is producing a whole lot of water flow in some areas (wickedly unnatural laminar streams) and still leaves calm or dead spots in other areas.
For bare-bottomed displays, you can often get away with this kind of flow as it just bangs around and becomes turbulent. But with fine substrates you need diffused flow. And high volume, diffused flow does not(!) disturb sands and is frankly better for the tank regardless of the use of substrate or not.
So... with regard for "disturbing" a biodiversity in living substrates that I say is weak as it is, I will concede that the occasional sand stirring or gravel siphoning of a sand bed can me helpful and may be necessary for folks with living substrates.
Its a matter of having to "pay to play"
If you like the look of substrates and/or want some of the benefits they offer... AND are realistic about their limitations, then by all means ... enjoy.
I'll also add (a gross generalization here) that hardcore Acroporid keepers usually fare better with bare-bottomed displays, and soft coral/LPS keepers fare as well or better with living substrates.
Its not an one vs. another situation, nor is it a "DSB" good or bad, "Bare-bottom" good vs. bad, discussion.
These applications need to be finessed on an individual basis to serve the needs of your individual system.
kind regards to all,
Anthony
Excelent thread with Anthony Calfo 
I Love My Sig By John Hawkins!!!!!!!!!!!!!
