I once saw them at a pet store when i was buying feeder shrimp and asked if i could have them and the store owner gladly gave them away. Later i found out they're used to filter water but i really just had them because it was interesting. However their very sensitive and one died then all died. Luckily when they died they unburrowed themselves.
BTW this was in a 20g planted filtered with a eheim 2215 so it didn't really need the clams to help filter
There are two famlies of freshwater clams.Sphaeridae and Unionidae. They are filter feeders and can help keep your water clean by filtering food particles from your water. Some require phytoplankton to survive which can be grown in a glass of water set in a window. Try these links for more information..
I have two clams, approx. 12-13 cm each. They're great, but they can uproot your plants if the roots aren't strong enough, so you may want to confine them maybe.