it would depend on species and size. some of the small clams need insane amounts of light as ther are quite un-efficient til they grow up a bit. which species did you have in mind?
Most clams would be fine under 6 x T5s id postioned correctly but Clams are very very sensetive and need to be researched alot before taking the plunge 1 tip is try and fid a clam od at least 4" this is not always an option though
PAR levels are dramatically decreased without individual reflectors (by almost 300%). I have also been researching higher light clams and SPS recently as I would like to keep one in my next tank and was looking at my lighting options. You would definitely be OK with Derasas, but I wouldn't be able to say for sure with Maximas. Maximas require PAR of about 200 if I remember correctly.
Yeah, Derasa clams ur going to be fine with. Since were on the topic of clams, this insane story was that I saw a guy's tank (local) and he raises baby maxima clams with compact flourescents, like the ones we use everyday for house lights. Crazy, huh?