The problem with keeping freshwater clams is they don't do much, just sit on the bottom and filter water.
They cannot be kept with fish because of the unpleasent way the clams breed.
When the clam eggs hatch, the lavae latch on to a fish and burrow into the fish's flesh causing a large painful cyst on the fish's skin.
Eventually the cyst bursts and the baby clams settle to the bottom and start acting like the adult clams.
In the wild it's not a problem because of the hundreds of larvae released into a river or lake by each clam, only a few will find a fish to parasitise and the fish will only end up carrying one or two larvae- stressful but not fatal in a healthy fish.
But in the restricted enviroment of an aquarium all the hundreds of larvae infect just a handful of fish killing them very quickly and painfully as they erupt in overwhelming multiple laval cysts.