I just want to say that if anyone has freshwater clams or mussels in their tanks, please be very careful not to allow them to escape into the waterways. One particular type of freshwater clam, the corbicula fluminea, is very invasive and breeds rapidly. There are other types that also breed rapidly as well.
Project attempts to smother invasive clams
In areas where it was accidentally introduced, it has literally choked waterways and displaced native species. Because this type of clam breeds rapidly, please be sure not to dispose of its offspring in any manner that could endanger your local ecological system.
Indigo, your clam/mussel may not be the type that rapidly breeds but I wanted to toss that out there for anyone reading the thread to be aware of the ecological dangers.
That said, I once thought about getting a freshwater clam myself. They are very good at keeping the substrate stirred up but they do require a deeper bed than normal, as they like to burrow down. They are filter feeders and unless your tank is seriously overstocked and overfed, they most likely won't get enough food without you target feeding them crumbled spirulina flakes or powder. A good way to feed them without polluting your tank is to use a turkey baster to squirt the powder/flakes close to them.
Enjoy your new pet!