Not a lot of clam experiance, but I am keeping a Maxima on the sand bed in my main display (30 inches deep) however I am running T5's. Now from what I understand the maxima needs much more light than the derasa. If you already have the clam you may want to try to partially bury a piece of rock in the sand and place the clam on it, then once it attaches (sometimes right away sometimes it take a few weeks) you can then move it higher in the display. Problem with this is if it doesn't like where you put it it will detach and move on you.
Good Luck