When you say at 22" depth I assume that is 22" below the surface of the water. That is pretty deep and I wonder if with even the 400watt halides if the Crocea is getting enough light.
I am by no means an expert, have only been keeping clams for less than a year but here are some of my observations.
I keep my three corceas in the rock with mantle at about 6" below the water surface. I am running two 175 watt MH and one 130 watt PC actinic over a 55 long. I tried keeping them lower but they would try to move and then fall from the rock. Since placing them high they have been happy and stay put.
Now my two maxima's, derasa, and squamosa are a different story, they all prefer the sand bed which equates to the mantle being about 14" or so below the surface. I tried keeping them in the rock work closer to the middle of the tank but they kept moving down to the sand bed.
A short period after I got my first couple of clams, a few weeks or so, one of them started to open less mantle and I noted some pinching in a couple of spots. I read several threads about pinched mantles and found the common cure was carbon. I started running carbon in a large canister and the pinched mantle cleared up. In fact if I forget to change the carbon (generally monthly) I will see a pinching or less extension of the mantle indicating it is time to refresh the carbon.
