I have to disagree with Bottomfeeder on this one--I think that clams are a good idea, but I do agree that they can be a little difficult to care for.
Freshwater clams act as supplemental filters in your aquarium and effectively lower your nitrate count. They can also act as instruments to measure water quality: when your clams begin moving around and can't seem to find a place where they are comfortable, more than likely your water quality is suffering.
When it comes to feeding, you can either purchase invertebrate food (usually a powder or grainy substance you can periodically scoop into the water) or you can give them "green water." Green water is simply water that is green with algae and microorganisms (what happens if you were to leave an aquarium under a light without filtration. Of course, adding these foods could hurt your overall water clarity.
Clams do great in aquariums with high bio-loads, especially with large fish--just so long as those fish don't eat them. Do your research to make sure none of your fish will make snacks out of them.
Don't forget to avoid using medications in the aquarium, especially those with copper. Clams and mussels are VERY sensitive to chemicals added to the water.
