Yep, they feed off organisms in the water column, they wont eat from the substrate. Someone said theres escaped in this thread. They do try to get high in the tank to reach the water current, so this is why they may have escaped.
Someone said that if you crush up some algae wafer really good it mgiht work for them but not long, they need a tank with lots of organisms in the water. They are just like clams, they filter feed, and thats why clams die too cause there isnt enough food for them and they starve.