I think there is a clarification to be made here, tridacnid clams are different from other clams because of their zooxanthellae. If tridacnids do not reduce nitrate and ammonia, why would commercial aquaculture methods add it to growout systems? Most clams are not that different from other organisms we add to our tanks in terms of food goes in ammonia goes out to be dealt with. However, the zooxanthellae processes the inorganic nitrate and ammonia (ammonia preferentially). Yes, the clam produces ammonia, but the zooxanthellae uses some of it. It is a question of out vs. in, tridacnids (with their zooxanthellae) use more nitrate and ammonia than they produce (which is why we so often hear that tridacnids greater than 2" need to be fed... as a side note, the tridacnids are not born with zooxanthellae, they need up ingest it, hence feeding smaller clams). What I am not sure of is how many clams would be necessary to effectively reduce nitrate in a home aquarium. I would venture to guess the number to be more than most of us keep; however, I agree that it is just another benefit to having these beautiful organisms in our system.
Here are some sources that state this:
http://www.sciencedi...ae03b46a3c3f873
http://animal-world....ms/tridacna.htm
http://etd.fcla.edu/...12200/alo_m.pdf