10,000k 250's are nice and adequate for clams. Or you could go 6500k and do actinic supplementation. IMO too many forget about the yellow green part of the spectrum which is essential for the zooxanthellae in the mantle and proper growth.
Since most (if not all) farmed clams come from very shallow water,
we want to emulate this lighting as much as possible. The only exception is gigas, who would appreciate more blue in his light and decreased intensity.
I am not one to provide more "blue light" than necessary. Furthermore, if you do use actinics to color correct, might I suggest turning them on and off at the same time as your other lighting. I know some people stagger the timing because it "looks cool" to have just actinics on, but the bottom line is, at least with clams, any sudden rise in color temperature is stressful to clams.
Simply because an animal's algae fluoresces doesn't mean it is fine and happy with a radical color shift in lighting. The clam god, Knop, also advises against such color shifts in his book. I agree with him 100%.