Freshwater clams come in two basic sorts: swan mussels and Asian clams.
Swan mussels (Unio, Anodonta, etc.) are basically impossible to keep alive in aquaria. They require lots of green algae (or some substitute) as well as cold, well oxygenated water. They are perhaps viable in ponds, but otherwise of no value to the aquarist. Wild swan mussels live for decades, even over 100 years; in captivity most die after a few months. Enough said really.
Asian clams are Corbicula fluminea. They are extremely difficult to maintain in aquaria. Again, they are filter feeders, and will need to be fed daily on a filter feeder food of the sort given to corals and tubeworms in marine aquaria. Otherwise they starve to death. They ARE NOT scavengers and THEY CANNOT survive without being given filter feeder food. They prefer (need) a sandy substrate into which they can burrow. When they are healthy and happy, they are hidden from view except for their fleshy siphons. If you can see the shell, then the animal is unhappy and likely to die. Asian clams usually die in aquaria quite quickly, and when they rot, they pollute the aquarium.
Bottom line, unless you are a very experienced aquarist willing to set up a dedicated aquarium just for filter-feeding animals, do not buy clams. Total waste of money. Do not release unwanted clams into the wild. Several species, including Asian clams, are notorious pests when released into the wild.
Cheers, Neale