Tridacna clams need to be kept in a well aged mature reef system. They are like sps corals in that respect. Barry suggests clams not be added for at the very least 6 months.
Currently I have 1 4" T. maxima tear drop and it is right up under my water surface, just off to the side of a 250w 10k XM. It has easily grown a good 1" in the time I've had it. I have 3 other Pohnpei collected maxima's from Barry at
www.clamsdirect.com/phpbb2/ They seem to be a much faster growing maxima. These are also high in my rock structure on the other off side of the 250w 10k. These 3 always show new shell growth.
I have 1 baby cultured maxima at 2" and it is also showing great new shell growth. It's fairly high in the rocks too.
As far as lighting for any of the tridacna clams..crocea have got to have halides as well as maximas. T. derasa and squamosa can very easily be kept under vho and or pc. Hippopus clams are also a great clam to keep under less than halides.