while there is a freshwater section on the reef tank, it's pretty much all saltwater here, I would also recommend
aquariacentral.com They are more dedicated to freshwater. I've gotten a great deal more responses on my freshwater questions there. fresh water clams like saltwater clams are filter feeders and they will filter nitrates, algeas etc from the water column. I've never had to add anything special to make mine thrive, other than NOT keeping the very clean. When the water was to clean and I filtered to much naturally I was taking away their food source and they started to decline in health. Unlike saltwater clams the lighting makes no difference so that's kind of a plus money wise.