How deep can I place my clam?

[Kkubiak]

I have a 75 gallon aquarium and my lighting consists of two 175W Metal Hallides 14000K. I also have two 110W VHO's, one Actinic Blue and one 50/50. Will it get enough light if I place it on the sandbed or should it be placed higher in the aquarium? The sandbed is about 20" from the lights. Also will glass canopies make a difference?

 

[jbittner]

place it on the sand and move it up if the mantle over extends or the clam appears stressed