Do you know whether these are actually clams or mussels, can you tell the difference?
Most of our native female mussels release larva after breeding that attach to fish, especially their gills, so it's not good to put random clams or mussels in your tank.
A large portion of native mussels are endagered, so it's illegal to collect many of them.
Asian clams are an invasive species and can generally be collected in any number (to eat anyway...check your states laws), but unless you can identify the asian clams from the natives you shouldn't take any. (Some states might have laws against keeping them in aquariums since they're invasive, but I'm not sure.)
As far as I know asian clams larva are freeswimming and not a danger to fish.