09-13-2009, 07:26 PM
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At a 4'' to 6'' sand bed you will achieve denitrification at optimal levels, which is why most people offer this recommendation. Unfortunately, this can be a considerable cost for larger tanks, so the other option is to use 1'' or less.
At 1'' or less you will achieve very little denitrification from the sand bed, so addition live rock is a benefit. However, at such a small depth you have very little risk of detritus accumulation, which would result in increasing phosphates and increased algae growth.
At depths of over1'' and under 4'' you still achieve very little denitrification, but the build up of detritus becomes an issue. At 4'' or more detritus rarely accumulates because of the life forms that inhabit the sand, feeding on detritus and food decay.
For the record, my 54 reef has a 4''+ sand bed. My 180 FOWLR has a <1'' sand bed. Both depths work well for me, and have for many years.