My understanding is this, there are some eels who when young can handle freshwater but as eels get older they, in the wild, tend to move to brackish (a mix of fresh and saltwater I believe around a 1.017 SG) and in the adult stages of life tend to move into saltwater. So to answer your question simply no a moray eel wouldn't do well long term in a freshwater tank, I would suggest going with a freshwater eel but even they prefer a more salty environment later in life.