These large clams are often interesting because of both size and colour.
Tridacna clams typically are easy to keep, if one has powerful lighting and good water quality. The calcium level of the water should be between 400 and 440 ppm, as this allows the clam to build up its shell. One should however avoid fish and invertebrates that might damage the clam.
Small clams (<4 inch or <10 cm) need a diet of phtytoplankton until they have grown. Larger specimens derive most of their nutrion from photosynthesis, but it is still beneficial to supplement with phytoplankton.
Tridacna clams will become attached, to varying extents, to the rocks or sand over time. Be careful not to damage the foot of the clam when moving them. Placing them in a ceramic bowl made for this purpose might be handy if you need to move the clam.
Pyramid snails often target clams, so make sure to check for these at the foot of the clam. Some types of Wrasses are effective at eliminating these snails.
When choosing a clam, it's good to check that it reacts suddenly to shadows by retracting. If it stays open or has trouble retracting there the specimen isn't healthy.