yes Higher intensity will give you better penetration but if he has low watts per gallon in a big tank that could be the problem couldnt it?
I prefer to think of it from the oppisite direction...A tank with 6-7 wpg
lighting would not automatically qualitfy it to keep light-demanding animals. In this case, depth is what is important. While 150watts of MH
lighting may not be enought to sustain a clam placed on the substrate. Moving the clam to a higher place on the rockwork is a better option.
Small clams neeeeeed to be fed. They need to be taken out of the tank (in a bowl of water) and the water needs to be saturated with phytoplankton (DTs).
I agree with Phyllis in that small clams do need direct feedings. I would caution removing the clam from the tank to do so. Rock dwelling species such as the Crocea will actually attach to the rock and romoving them could cause damage to the foot...leading to infection etc. Even clams that prefer the sustrate should be provided with a flat rock burried in the substrate that they can attach to.