clams

they need lots of running fresh water its not a good idea to put them in a aquarium it would get very nasty

They don't need running water, but what they do need is very good water quality and a microscopic food source like single cell algae. They're basically a living filter and will clear a green water outbreak inshort order. If you have or can start a green water culture they're not difficult as long as you keep the water slightly green all the time. Personally I find them pretty boring since they usually bury themselves leaving just their siphons exposed. Unless I was breeding Bitterlings I personally wouldn't bother with them in an aquarium. If you have an outdoor pond it would be a perfect place to keep them.