typically, there are two species commonly sold as the freshwater moray eel, Echidna rhodochilus and Gymnothorax polyurandon. as juveniles they can be kept in freshwater, although both are true brackish fish, which also do well in full marine conditions. neither species should be kept in freshwater for long term health.
they grow to about 30 cm. (12 inches) and if you're going to house this eel with other tankmates, a 75 gallons is appropriate.