I think it also depends on the clams.
Derasa and Squamosa will be happy on the sand and thus less light requiring. Maxima and Crocea would have to sit higher, prefer rock and require more intense lighting.
I grew Maxima at a very fast pace under a 5 bulb ATI fixture.
If you are a charter member consider checking our library on MAAST. We have a great book on clams available.