Nothing has changed as of too recent. Let me think here...
I added a kalk reactor about a month ago...but have been tuning it a bit still to get the output just right. For a while right before the clams shut up, there might have been some 'kalk-clouds' getting released directly into the tank. My pH was still 8.5 when I checked.
I havent added any new fish, inverts, or anything 'live' in some time. I did buy and feed some DT's right before the mantle problems started.
My tank was going through some cycling issues right before this, but my corals have colored up, organics and phosphates are nil, and algae disappeared. I was using phosguard, an Al based phosphate remover, multiple times prior to this with no problems. I would rinse it well before using so that the dust wouldnt bother anything. FWIW, I had forgotten to replace the phosguard when cleaning out the old stuff just days before this started. My phos readings were nil so I wanted to see how things were without it. So for the period just a few days before the clams went into hibernation, I wasnt even using phosguard like usual.
As of today, my girlfriend said that they alll come out during the day, but they are very sensitive to people walking by and so they shut up when I sit down. Two seem better, the other two are still mostly shut.
I have looked for snails, and dont see anything. I did a 50% water change 2 days ago to dilute any chemicals that might be irritating them. I also did a flatworm exit treatment right before the water change...and my skimmer was pulling out red gunk...whole bodies even, for two days. Neither of these two things seem to change anything as far as the clams care.
OTOH, I have noticed some light patches on some SPS in the tank. Nothing major, no polyp damage, except for a bleaching of a spot or two on a few corals about 1/4" round. Except a purple digi which was very close to the kalk outlet. Shortly after the clams went into hiding, this one piece (I have it growing like a weed elsewhere in the tank, but only this one was like this) had bad bleaching in almost 50% of its body (about a 20 point piece). This piece was close to the surface, right under the halide, as well as the kalk outlet.
I have since moved the kalk outlet into the sump just in case it ever bleeds pure kalk again. The kalk reactor has also been tuned just right now. Right now, for lack of a better reason, I am going with 'kalk-burn'. Which is funny because I have mixed so much it turns into snow in the tank before, and the clams have never been bothered. But Im thinking what might have happened could have been much worse when I wasnt looking.