Never actually heard of this working for prolonged periods. Those I know who have tried it (including me) had no luck with long term keeping of these critters. They are not meant for reef tanks. The clams sold as food are usually from cold water regions and not from reef zones that we are trying to simulate.
In every case I have heard, again including mine, is that they died shortly after introduction (a week or two) and actually created an ammonia spike and fouled the water.
There are usually not enough for these clams to filter out of the water, and I suspect they die from starvation as well.
This is not a method that should be used, IMO, as a way to control nitrates. If you need to get nitrates under control, get a good protein skimmer, make a fuge with cheato, or do water changes more frequently.