Originally Posted by onefish2fish
the dept of your sand bed is very important. I would go 1'' or 1.5'' OR 5-5.5'' to 6'' deep. Any depth of sand in between traps detritus and debris and causing more problems.
Wow. This rarely happens. I very respectfully disagree on this. It is a very long topic and Goemans has a ton of information available on the web, but here is the quick logic...
Small amounts of sand, say 1.5'' or under with good sand sifters is ok, but you would be better to go bare bottom. Any amount of sand will trap detritus, and you need a deep enough sand bed to allow for denitrification. 1.5'' won't hurt, but you have to be very cautious of water flow and detritus accumulation.
The old rule of thumb to use 5-6'' of sand came from people attempting to use a modified plenum system, without a plenum. They would place 2'' of crushed coral, followed by window screening, then another 2'' of crushed coral, then another layer of window screening, and finally 2'' of aragonite sand. This worked ok, but if the bottom layer was disturbed a hydrogen sulfide smell would pollute the tank, because the sand be was to deep. The correct method was to use 5-6'' minimum placed on top of a plenum, used to raise the sand bed. This allowed waster to slowly flow thru the sand and eliminate dead layers.
This works fine and is the "plenum" method of denitrification.
The other method is referred to as a "Deep Sand Bed", or DSB method. This is the method I personally have used for a decade or longer and have discussed many times on this site. You want a MINIMUM of 3'' and preferably 4'' of aragonite sand. If you go much over 4.5'' of sand you will get dead layers as described above. The DSB works well provided you have adequate amounts of live rock to "seed" the sand bed, and sand sifters to keep the sand bed gently stirred. Usually hermit crabs, snails, and starfish are sufficient. The amount of life that thrives in the sand bed is very similar to having a Refugium, as the fish will not eat the copepods, amphipods, etc etc until they come to the surface. These small lifeforms keep the sand bed from packing and allow for extremely effective denitrification.
My aquarium had a Nitrate cycle very similar to a typical Ammonia & Nitrite cycle. The Nitrate climbed to 20ppm after 6 weeks and by week 10 was zero. It has remained zero for over 8 months with 3 daily feedings. This is typical of my experience with these types of setups.
By the way, I sometimes sit for extended periods of time just watching the littler critters in my sand bed. It is amazing to watch how much life actually exists in these aquariums.