i have the same tank and similar lighting and never had any problems with my clam. it has been happy from the start. light shouldnt be an issue the clam should get plenty of light at any depth. assuming 750w mh means 3 250w.
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125 gallon glass. 3 10k 250w metal halides, 1 4ft 110w vho actinic. who knows how much live rock. yellow tang, domino, cinnamon clown, percula clown, flame angel,peppermint shrimp, camel shrimp, skunk shrimp, a few turbo snails, lots of hermit crabs, leather coral, branching frogspawn, mushrooms, too much xenia, and several different zoo's and button polyps | 
