First of all, make sure that your aquarium has been established long enough to support the needs of your new addition. A good indication of your aquarium’s suitability for Tridacnids is a nice growth of coralline algae. Such growth usually occurs 6 months to a year after your aquarium has been established.
Make sure that your fish and invertebrates are compatible with Tridacnids. Some cleaner wrasses have proven to be destructive to clams, nibbling on their mantles and otherwise preventing full expansion. Other fishes may also harass clams. For example, I once had a Copperband Butterfly that was introduced into an established aquarium that contained clams. It never bothered the clams at all, until one day I placed a few new clams into the aquarium and he attacked them within moments. Observation and careful selection of fishes is critical if you tend to be successful with clams.