Well, I think clams can survive with less light than what is posted. I had a 55 gal when I got my first clam. A Squamosa, which is a lower light clam, but still when I bought it, it was about the size of large egg. It was under two 65 watt 50/50 PCs. Grew like crazy. I have since upgraded to a 180 gal and the clam is under 2, 300 watt 10000K MHs. About the size my head now. The clam was under PCs for a year and have been under the MHs for a year. Better growth under the MHs, but as long as you keep your ALK and CAL up to par, the clam will growth fine under PCs.