they will turn brown because of nitrogen levels being to high in the water regardless of lighting they get(MIKE PALETTA talked about this when he talk at the cora meeting, talked about the waikki aquarium having this problem with there clams and corals, high nitrate made them turn brown and it wasnt lighting because they use natural lighting)
just wanted to share that not trying to step on toes, we know clams and sps require high lighting to be kept healthy and happy
FWIW
ben