DSB in a sump

[fatman]

What causes the clumping/crusting of the sand bed?

There typically is no clumping in a remote deep sand bed if light is eliminated and detritus is not allowed to settle. The deep sand bed will mainly remove nitrates (Denitrify), and will also dissolve, there by suppling calcium, carbonates, magnesium, and stontium. Your live rock in the tank will provide nitrification and appearance. The live sand will do nitrification , but without the settling of detritus and other solids organics the main nitrification will be done in the display tank by the live rock. Remote deep sand beds are best utilized for keeping nitrates at zero, and this they can easily do. A remote deep sand bed of 100 inches square 10 x 10 inches that is 8 to 10 inches deep can easily keep 50 to 60 gallons of well stocked reef tank at a zero nitrate reading with a two inch level of water over it with a flow of 200 to 250 gph. Or 14 inches x 14 inches for a 120 gallon tank. That is with no live rock covering any of the surface, no lights (therefore no diatoms or other nuisance algaes) and no critters (not needed without the settling orgainic), just sand.

 

[fatman]

What causes the clumping/crusting of the sand bed?

There is a good article in the "setting up your tank" section called "more on sand beds," where the last third or fourth of the article is about sand clumping. In reading the article you should see that a remote deep sand bed, without lighting and critters really does not have clumping problems as the chief sources do not come into play in remote deep sand beds as descibed earlier.