Barbara;604474 wrote: Two 150s as in 2 metal halides? If I recall correctly, the 72 bowfront is about 24" deep (top to bottom); correct? 150 MH is a stretch for 24 inches, but you should be able to observe the clam, notice if it's not happy (mantle not extending properly), and try to move it around a little. Just remember to put it on an inverted shell or another rock for support in case it decides to attach it's foot. And also be aware that they can "jump" if they are on a rock ledge and not happy, so be careful if you move it up on the reef.
I think ideally clams should be on the bottom. It prevents them from attaching to your reef and also keeps them from "jumping" and landing upside down in the sand or on an SPS. However, you have to have enough light reaching the sandbed. Maximas require a bit more light than a deresa clam (if I'm remembering correctly). Just keep an eye on it.
Good luck.