When I did my major tank reorganization a few months ago, I had planned on moving my dersa and maxima with the 250 MH lighting. However, I have been unable to move my coris wrasse out of that tank, so for the past two months the clams are living in a 48 gallon bowfront tank with heavy VHO and PC. (I also keep a carpet anemone in this tank.) The clams look great. In fact, they look a little better than they did when they were in the other tank - better mantle expansion. However, I fear that is due to less light. I have been removing them from the tank and feeding them two times each week. The dersa has white rings and is growing. But not as quickly as it was growing under the MH. It is in the middle of the tank in the rockwork. The maxima has never grown since I got it - even when it was under the MH. It had a little bit of a rough start - the purple tang kept bothering it. I keep it 3 inches from the top of the tank which is about 10 inches from the lights.
I do not recommend this. I think the clam can live but not thrive. I am planning at this point to buy a 250 MH for the other tank when the current bulbs age out.
Just my experience so far!
Connie