I can't speak to your lightng, altho T. derasa is one of the least light-hungry of the Tridacnids. One thing I would consider is feeding the clam live phyto a couple of times a week rather than leaving everything up to photosynthesis alone. Clams have a fully-developed digestive system, and appreciate regular feedings. Try not to use Nannochloropsis as the cells have a thick, tuff wall that isn't digested by most organisms. Isochrysis and Tetraselmis are much better and have better nutritional profiles to boot.
One thing i CAN offer is regarding clams and flow...they don't require, or do well when being blown around. Anything more than an occasional gentle lifting of the mantle will likely result in poor mantle extension and stress. So be sure the clam is placed in a "gentle flow" area of the substrate.