Justin had an interesting post during this thread, a topic I have been looking into- the over dosing of alk and calc, which then calcifies in the sand causing bricks/ clumps of sand. The calcification causes the release of hydrogen sulfide which is a coral killer, especially sps but could be problematic for hardier corals in high volumes. I think that I had this issue at one point. All levels of alk and calc, ph, phosphates etc are fine, no pests, but a slow sps recession. I noticed earlier that I had some clumpiness to the sand, some larger clumps, and some that broke apart. Its something I had no idea about until Justin mentioned it in this thread and I followed up. Could be a possibility Duke if you noticed any clumps.