[pogodzib]

Tridacna derasa, squamosa, and gigas are best placed on the sand, whereas crocea and maxima should be placed in rocks. Clams do need a lot of light and you'd be pushing the limits on keeping a clam in the sand for a tank that tall with your lighting. Your best bet would probably be to keep a derasa or squamosa if you want to keep on in the sand. The best way of keeping one in the sand is to bury a flat piece of rock 1"-1 1/2" under the sand where you want the clam to be. This way, the clam should attach to the rock under the sand and stay put and be easy to move if you need to move it in the future.