any clue why clams start to fad in the center?

Central bleaching - Daniel Knop has detailed this in his book. If your lights are right for the clam, it could be due to the lack of an essential element. How are your CA, ALK, MG, salinity, etc?
 
it also happens to clams when they're not getting enough even spectrum lite. Guys who put them under too much actinic etc... Also can happen to one side if its shaded by the rest of the mantle or siphon. It'll usually come back if you correct the problem. I got a very bleached clam from a guy w/ pcs & under MH it came back in about 6 months...
 
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