While some have had success with hardier Derasa clams under 4xT5 lighting, its not a sure thing. If you want to keep a Tridacnid Clam, I'd say for sure you would be ok with a 6xT5 setup or a moderately powerful metal halide lighting setup. One thing to consider for sure with Tridacnids is that unless you have 400watt halides on there, do not purchase a clam under 4" in shell length. Under that size many clams' vascular systems (which house zooxanthellae) are under-developed and the clam therefore requires significant light to make up for the lack of zooxanthellae.
