From Keith @ Marine Depot:
Generally you will find substrates clumping togther when you have a combination of high alkalinity (generally above the recommended levels of 9-12 dKH) combined with a high pH level. This combination allows for the fomation of calcite crystals. That is one possiblity of what is happening to your substrate.
You can also find if you have high organic levels in the substrate these can cause it to clump together. Alga and detritus can cause something called "organic bonding".
Lastly if you have high phosphate levels it is possible they are precipitating out in the substrate and causing the substrate to bind together.
Double check your alkalinity levels to make sure they are where they should be, make sure you pH is not going above 8.4 and if it is a chemical binding of calcite taking care of those two water parameters should help. If it is organic bonding siphon cleaning the substrate about every 2-3 weeks should help out along with nutrient control (not over feeding). If it is a problem with phosphate binding, then using a phosphate remover media such as Rowaphos or Phosban to help pull the phosphates out of the water will help.
Chuck, I think this what you were asking for.
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Rico's Reef - Born 07/15/2006 - 75 gallon reef, 29 gallon sump/fuge
Polly's Ocean - Born 11/24/2007 - 10 gallon mini reef
Rico's Moneypit - Born???(coming soon) - 240 gallon mixed reef
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