Rob,
Your lighting is adequate, but there are other factors. Some other inhabitants may pick on a clam. Has your tank fully cycled, and are ammonia and nitrite down to zero? Skimming?
I would not recommend adding a clam to a tank that does not yet have any other inhabitants. You want to be sure that everything is stable as clams are pretty sensitive.
If you have some other critters in there, and water quality has been consistantly good, Derasas are good first clams. Try to get one at least 3", and beware that it will grow fast, demanding alot of calcium, and could outgrow your tank in a couple of years.
Anyway, good luck, and be patient. Most 7-8 month old tanks probably aren't ready for clams, especially if this is your first tank.
Adam