Dana - my clam is under LEDs and has done absolutely awesome - before the Majestic Angel decided that clams were a food source :frustrat:. I took a clear tupperware container and poked a few holes in it and placed it over the clam, hopefully he will open up again and not starve
Anyway, as far as where you place the clams, that depends on the clam, and probably the lighting as well. Derasa and Squamosa clams like the sand, and are found in much deeper water than maxima's or crocea clams. Also, if you get a larger derasa clam, they tend not to attach to anything with byssal threads, which means they can be moved around the tank if need be.
This is totally off subject, but there is a LFS downtown called Best Fish, if you havent been there yet, you should stop there the next time you come up to Aquatics Unlimited. After finding Best Fish through the local reef club, I dont even shop at Aquatics anymore. If you're interested PM me, I can send you the address and the link to thier stock list