I have read somewhere and confirmation would be better rather then taking my word on the subject, however I had read somewhere that clams can actually promote health in a tank or a.k.a act as a sort of additional filtration because they consume some nasties in the water, now I am not saying that a clam can be your filtration for your system. What I am saying is that I have heard of them contributing to healthy tanks rather then just consuming the good stuff and releasing more nasty stuff into the water.
Confirmation ?