FW clams breed by releasing egg and sperm into the water, where they meet up and fertilize. The resulting wriggler then attached to a fish--many are species specific--where they feed for anywhere from a few weeks to several months before dropping off and attaching. They develop their shell at this point.
Sorry, but it's really not likely to breed them in an aquarium. IMO, they are not a good aquarium specimen, since they are very sensitive to water quality and it's difficult to maintain enough food in the water column for them without compromising water quality.