Well whatever it was is not gone. I checked this morning expecting to see an empty shell and the clam was there looking normal. There does not appear to be anything coming out of the siphon anymore but the lights were not on so it was not really open yet.To answer the questions from above.
Tank - 125g AGA - upgraded from 55g in Feb.
Lighting - 3 x 250 MH - XM 10k bulbs. 1 new in 10/08 and 2 new in 1/09 - 2 x VHO Actinic
Flow - 2 x Tunze 6000 with 7091 controller. Each alternate between 100% & 30% flow rate. Max flow rate per pump is 1849 gph. Return pump is a Rio 32HF with a flow of approx 1200 gph
Parameters - 4/4 SG: 1.025; pH: 8.2; Ammonia: 0; Nitrite: 0; Nitrate: 0; PO: 0; dKh: 7; Calcium: 480
Clam - Bought in July 2008 @ 2"; Today is approx 3"
Of note: The clam has never secured to any rock. Always has prefered the sand bed.
Thanks, J