As I recall, we have very similar set-ups. I am using 4x 400watt Iwasakis and 3x 6' VHO actinics over my 78"x32' tank. The tank is 37" deep with a 5" DSB. My problem is usually too much light, not enough. And in particular, I sometimes have a difficult time getting sps and lps corals to adjust to these light levels, even if I strat these corals out on the bottom and move them up slowly.
In any event, I am just starting to add some new clams. I have a large squamosa that is doing very well on the bottom and a maxima that is near mid-tank. I have had the maxima for a while, and it has gotten much darker since I got it, but it is almost a dark brown/black at this point. But I just added a crocea and I placed it pretty high in the tank (about 12" below the surface). It seems to like it there.
I plan to add more croceas and maximas, but I don't have a good sense as to whether they need to be high in the tank, or if they will be ok on the bottom. Placement high in the tank is limited since it can be tricky to place larger clams up on the rockwork in such a manner that they won't fall. Thus, I would rather place the clams on the bottom if there is enough light.
