Do you know it's history?
Because we may not be sure how much light it is accustomed to, start it off at the bottom of the tank. Some aquarists gradually move it upward, a few inches every week or two. Great care must be taken to ensure the clam does not fall off of it's perch. Some have used small glass ashtrays filled with substrate to act as a clam bed.
Once the clam's byssal gland or foot has securely attached to something, it is best to leave it alone. It can reside quite happily on the bottom, for it is one of the less demanding clams after T. derasa and T. gigas.
When the Tridacnids are tiny (under two inches), they are aggressive filter feeders. After they are about 2.5 inches, they rely almost solely on the photosynthic zooxanthellae in their mantle. Phytoplankton is not necessary, nor other foods (but I am sure some of your other critters would benefit from tiny periodic doses!).