Just a note about aquarium guys ,they are a good little company..not sure if you used them before or not...i get all my supplies from them..just got a second nova t-5 from them...
The Current Nova Extremes do not have individual reflectors.The Current Sundials(4x39w) and the Current Nova Pros(6x39w) are the ones that do have individual reflectors.
I would not try any clams except T.derasa under the Extremes.Clams mid-level or higher with the Sundials or anywhere with the Pros.
Sure,I think a derasa clam will do fine with the Extremes.The Nova Extremes from Aquarium Guys is $238.89.The Current Sundials at Dr. Foster and Smith is 239.99 and at Premium Aquatics it's 259.99.The price difference is small to none,get the Sundials that have individual reflectors.Hey,it even has timers built into them.