SAMZ,
My clam holding tank is a 60 long (17")tall with 80lbs of LR and eggcrete about the middle of tank to be closer to HQIs. Have a 29gal lemar for a sump with sand in it so I can place small T. derasa and squamosa and have 2 PC over that tank.
Am looking at a square tank, all open top 36"x36"x24" for my addtional holding tank. Most likely run a few 400ers over it with some actinic suppliments.