^^. Clams need pristine water in terms of nitrogenous waste products. But they like dirty water in terms of water filled with lots of microorganisms for them to feed on (unless they're tridacnids, but those are saltwater photosynthetic exceptions).
Have you tried using a plant filter, specifically something like a Pothos? I've heard good things about Pothos plants just sucking up all the waste products (but not the detritus) in monster fish tanks.
However, if you were to seed the water with a freshwater microalgae species, like Nannochloris or Scenedesmus, and you had a resultant green water bloom....you might (heavy, heavy emphasis on might), be able to keep one clam/mussel. It would most likely not work, and would probably only work for a very large system and one clam/mussel.
So many fish/plants/inverts to keep, not enough aquaria.