I'd hold off on a clam for another 4 months or so, especially if this is your first tank. They can be somewhat delicate, and you really want a more stable environment, which you're just not going to have with a 2 month old tank, even if it may look like it right now. Then, you'll want to look into what type of clam you want. Your lighting should be sufficient for just about any of them, though, so you're off to a good start there.