FW clams need a food supplement to live for any prolonged period of time in a normal aquarium setup. I feed mine, who are by themselves in the winter in a 6G tank, zooplankton. In the summer they go in the pond where there is plenty of stuff for them to sift.
They are not a good member of a clean up crew in FW. Usually what happens is they starve to death and rot under the sand, ammonia spikes, fish get sick and possibly die. Best to avoid them in these setups, though they are amazingly good at clearing up pea soup algae.