Don't get one. Buying one is basically condemning it to a slow wasting death, and supporting the trade in a creature that really isn't suited to home aquariums.
Unlike many of the prettier clams, they do not have any bacteria to help sustain them: lighting is irrelevant. They are filter feeders, with extremely picky feeding requirements of foods that are almost impossible to sustain in high enough quantities in a home tank. You have to spot feed them to have any hope of keeping one alive, but they scare easily and so hide in places that are hard to spot feed them, and hard to see them in any case.