Pete,
I am using many different types of lighting with my clams. I am also tracking the growth rates and ect. I am using MHs: 400 watt 10,000ks, 15,000ks, 6,500ks, Pcs 8,800ks, 10,000ks. I supplement all with actinic lighting which is vho, no and pcs. I also am using a 150 hqi 10,000k. I am in the process of also setting up Mhs of 175 watt and 250. All the lights are the same distance from the clams. I had custom tanks made for my clams. They are 6 fett long by 18 inches wide by 16 inches hight. Using a 50 gallon sump gives me a great system. I also use about a 4 inch bed of gravel. The picture here is a 400 watt 15,000k bulb.
Jim