Q: Blue Maxima Clam Relocating

i have a 34x34x24 tank with 400watts mh on it now i added a maxima clam,
which was placed on a mantal on top the clam kept moving untill it hit the
bottom, it was at the bottom at the lfs should i bring it back on top of
the mantl or just leeave him be?

he's quite open and color is great where he is, i'm planning on adding
another 400w in the next month which would be a 20k and a 15k 800watts on
34x34" surface with a 24" depth hows that for lighting?

Thanks