mine is also happy under 250w of MH in my aquarium, very happy. hes on the highest rock in the aquarium and hasnt moved. already attached to the rock
i went over this at -- in the tridacnid forum.. minimum lighting for maxima and crocea in a 14 biocube, everyone said 150w of MH, unless you have a lot of PC lighting
you could try crocea since max size would be like about 5" and grow slowly, but you need to make sure youre on top of the alk and calcium and make sure you have lighting to support them whether its mh or a lot of pc
its like all those people who say to get mh for anemones, but people ignore it and end up getting it with pc anyways
if you get a clam look for one that's at least 3in to make sure it has a higher survival rate