I had 4 fresh water mussels for around 8 months, 1 died last week, I think we left it out of water too long. apart from that they have grown a fair bit sense I got them or my hands have shrunk.
My friend who had them said the biggest issue is many carry a bunch of pests / diseases with them so they have to be QT for a long period and there no way of telling when its safe, I dont think they do anything amazing for the water personally but who knows.