For what its worth if you are keeping sps coral then you have enough light to keep any clam and that pinched look could be from several causes . Any dwarf angels or blennies in that tank or some type of clam eating mutant Tang ? I have had a few to acquire a taste for the old clam a purple and a yellow and even a powder brown all liked to nip on them... But if you are keeping corals then lighting is nit the issue I have kept clams under VHOs for yrs you have something bothering that clam either an internal pest infestation or and externial irritant or water problem .. Or the issue could be the shortened photo period ? May want to jack that up to more hrs a day no reason to lower the lighting for a SPS coral under any T5 set I have ever ran across . Unless it was under a weak set of T5's to begin with and by weak I mean just sufficient light to keep a SPS alive .. Any SPS I would ship out was under several 400 M/H with UVL VHO actinics and I mean under the light like this tank here a 90 has 3 400 20K and 2 48" VHO's I do have the clam about mid way in the rock work . Sometimes it more then PAR values that matter. Just went and read through this thread I see you have a flamtail blenny , Is that the dark blue or black with a yellowish dorsal and tail fin ? If so I have seen those and the salifin and Bi colored blennies to tear the living hell out of sps and Lps and clams .. although they are supposed to be vegetarians . And looking at those posted pictures of the clams decline over a 3 day period ? I would ask did you get this clam off of the Internet and it was shipped or from a LFS and if so how long had they had it and where was it setting in their tank and how was it moved to be bagged up ? For a 3 or 4 day period the clam show some serious decline from simple lack of lighting . And the fact you mentioned you had to move a SPS lower to get it used to the lighting suggest to me your lighting is not the cause . T. Crocea clams are the most delicate clam's we deal with in home aquariums . I would not suggest one to be placed under anything but M/H's myself . Also they are very delicate to handle and placement out of the LFS and in your tanks . They can be harmed and take months to decline and die . So your clam may have been damaged in handling or shipment even?