Currently what I'm keeping are freshwater mussels that I caught off the river.If you are still wanting something molluscy have you considered aquatic snail species? All the fun of the big land snails, but not constantly cleaning up slime trails, and you can put other stuff with them. I have a pomocae maculata, (scientific name for big nasty apple snail) and these boys get HUGE. My snail's shell is about the size of a golfball, and he's still growing. I keep in a 10 gallon guppy breeder tank with some bamboo shrimp. They all get along great, and the snail is so fun to observe scooting around the tank.
They look like these:

I have two at the moment and are 3inches long. I know they're not clams since clams like a perfect shape persay. While the mussel have a longer area on one part of the shell... I'm somewhat fascinated by it. Currently I'm trying to raise phytoplankton outside(greenwater), through different methods. Putting a few jars outside. One with fertilizer(it's already STARTING to look green), another with a dead fish in there(I haven't checked that one), and the rest are just water with food i n them or just plain old water and snails.
I have the mussels in a 2.5gallon tank. They say they need a 10g but they don't move alot regardless. LOL. I have a small 3gallon filter in it working at half the power(ill explain soon why).
I also put in there a ton of duckweed, and a crayfish. They duckweed oxygenate the tank, while the crayfish produces CO2(small ammounts), and while the clams filter out the water.
The filter is at half capacity because i don't want it to push the duckweed down and harm them. So the water is kinda still but it still has a TAD bit of movement. I turn on a weak powered air stone occassionally to move the water for the mussels to filter it. I kinda want actual clams and not mussels. But most clams for sale(freshwater) are small growing. I'd like one with a long lifespan, very nice looking and i'll be able to see.