Tridacnids are indeed filter feeders, and they will capture and eat a variety of particles. These include various sorts of phytoplankton and zooplankton, as well as minuscule bits of detritus.
Detritus is typically plentiful in aquariums, as it is primarily composed of fish waste and tiny bits of uneaten food. There are several products available to hobbyists that contain preserved or even live phytoplankton.
These products can be used to feed specimens, but tridacnids also have their zooxanthellae and can absorb dissolved nutrients directly from seawater, as well. Thus, feeding giant clams is unnecessary in well-stocked tanks. As long as there is a good number of fish, there will always be detritus, and the ammonia given off by fish as a waste product can be absorbed and used by tridacnids. Many hobbyists successfully kept numerous tridacnids of all sizes in aquaria before any sort of Phytoplankton products became popular or even available in the hobby.
Still in the case of one or more tridacnids being kept in a very sparsely populated aquarium, there may not be enough food and dissolved nutrients present, making feeding a requirement. This would be an unusual situation, though.
Basically, if all other conditions are acceptable, a specimen should grow, which can be seen when new white shell material is added to the edge of the shell. If it does not and you have a very small fish population, use a quality food. Follow the manufacturer's directions for use, and see if it makes a difference.
Conversely, you could also use food from the start, and then if you can see shell growth, you may want to cut back on the amount of food being used and see if the specimen continues to grow. If it does, you can cut back more and more until you are satisfied that no food is needed, then quit using it altogether.
