All 3 are up on the rocks - approx. 12" below 400W MH lights. I have not moved them in probably 6 months, except for one guy that fell off his rock. The one that I noticed this on first seems to have become unattached to the rockwork which has me thinking that maybe something is picking at their feet. My other 3 clams are down in the sand and they seem to be doing just fine - fully extended and looking good. I have not seen any fish picking at them and I cannot see any pyramiddal (sp) snails on their shells. I don't know what else I could test for as far as water quality goes. I did a phosphate test and it is about .03 with a Salifert test kit.
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