I believe that the experts say that at about 24" in depth, you will definitely need MH
for the survival of the clam.
As for starting off on the sand bed, this is IMO
a good idea if you are running MH
lighting. This way the clam can acclimate to the light. Once it attaches to the small flat piece of rock, you can move him up in stages.
Also & 2", you will need to supplement feed him with either DT's phytoplankton or Knop's Koralvite F. If you get the Koralvite F, watch out, it's super concentrated so a little goes a long way.