I would like to put in my 2 cents on the Deep Sand Bed issue; how it works, what purpose it serves, and why/why not to incorporate one on a marine tank.
In small tanks, less than 30 gallons or so, a DSB can be created from 3 inches of sand. As the tank size grows, a deeper sand bed is usually needed, usually 4-6 inches or deeper. My understanding of this is because of fluctuations in the depth of sand across the tank and the ability of oxygen to get to the bottom, among other things.
The other thing that must be considered when deciding your DSB depth is that the half life of aragonite sand in a healthy aquarium with no additives besides water changes is 18 months. This means that a 4 inch DSB will be 2 inches in 18 months. So that must be accounted for when implementing a DSB. I would err on the side of caution and make sure my DSB was starting around 5 inches so that I would not have to be adding sand to my tank all the time(dirty, unhealthy, and time consuming.)
The best sand for making a DSB is sugar sized aragonite. Smaller sized grains will create dust storms and many times be too tightly packed for much larger(still very small) life to thrive. Larger grains do not hold enough surface area for bacteria, hold too many large chunks of waste, and do not become anaerobic as easily.
A DSB is used to most frequently to complete the nitrate cycle so to speak.
The top portion of the DSB is aerobic and houses lots of aerobic bacteria that break food, ammonia, and nitrite down into nitrate. This portion can be carefully stirred occasionally, but no more than 1 inch down.
The lower portion is what makes it a DSB and is anaerobic. The bacteria has no oxygen because the water that gets here has had the aerobic bacteria use it all up before reaching this far down. The anaerobic bacteria breaks down nitrate into harmless gases that are released into the atmosphere. This portion CANNOT be disturbed because it will kill the anaerobic bacteria and make the DSB useless. Do not stir your DSB!
DSB's can be very beneficially in systems that have pretty good water parameters already, aren't overstocked or overfed, and have very high water flow. They can completely eliminate nitrates in a system that is properly stocked. The larger the surface area of the DSB the better as there are more bacteria breaking down wastes.
I must state that water changes are still necessary even if nitrates are at 0 in your tank. There are many things we cannot test for in the water and removing those wastes is necessary to keep the tank healthy. A DSB can just make them a little less frequent and keep your system running at a higher level in between them.
My 210 gallon tank has had 0 nitrates since incorporating a large remote DSB and I used to have a quite large Gray Bamboo Shark(the one Fleshy now cares for).
The problems with DSB's most frequently lie in being placed into tanks that are lacking in one or more of these areas. The build up of wastes in the DSB that can't be broken down fast enough can and will ultimately cause more problems than the benefit of having it in the first place. The waste (uneaten food, animal waste, dead organisms, etc.) will be trapped in all the sand in your tank and continuously decay causing water quality problems.
Included in these problems can be ammonia, nitrite, and high nitrate in your water which is what you were trying to get rid of in the first place.
The other problem that people have is a build of hydrogen sulfide. This causes the sand to turn dark black and if released suddenly will wipe out the entire system. This is the reason most anti-DSB people are such.
Only experienced aquarists with lightly stocked tanks, solid feeding habits, and good maintenance schedules should have a DSB within the main tank. In a reef setting it may be practical, but the remote DSB will create food for the corals and take up less viewing space in the display.
The best way to have a successful DSB is to put it in a remote location. This is a much better choice for almost everyone than in display DSB's.
A remote location would be in a sump/refugium. This way uneaten excess food and animal waste are not being deposited directly onto the DSB. That stuff is in the main display tank where it can still be siphoned because there is no DSB. You can place some live rock onto the DSB to help seed the it. The rock will also provide additional filtration and create a location for copepods to grow/multiple without any predators present. This is ideal for reef tanks, as you are supplying natural food sources for your corals. Macroalgaes can also be incorporated here. They can then be harvested to remove nutrients. The water should be fed directly from the overflow unfiltered, so no sponge, socks, sleeves, skimmers, etc. so that the DSB and macroalgae can use the nutrients in the water. This works best if you have a tank with two overflow tubes because you can send one tube through the refugium and another straight to the protein skimmer.
I believe that a remote DSB is a wise addition to any marine tank that is in otherwise good health. The DSB can make that tank incredibly healthy and the livestock within it very healthy and happy as well.
I hope this clears up a lot of the confusion on the DSB front. Hopefully I covered everything in the book I just wrote. I guess I ended up providing a lot more than my 2 cents....
