OK, now that I've learned through massive questioning and trial and error, I am familiar with freshwater morays.
Firstly, they are not freshwater as per say. They are saltwater and the only time they are freshwater is when they spawn, they then travel back to saltwater environments. The ones you get in your lfs
are young and are forced into fresh and brackish water, they would never live there in the wild until later in life.
They do get rather large (3 feet plus), but will grow relatively slowly to their environment. Recommended tank size is 20 gallon and up. I currently have one in a 15 but plan to expand to a 33 gallon. They will tolerate each other, but do better on their own, typically a lone tank species.
They do a lot better in a brackish tank setup with a specific gravity between 1.005 and 1.010. The temperature for these guys is a balmy 80 degrees. Be sure to make a cave or lair for them because they need to hide. To make a cave I used a large margarine container cut in in half with a hole in one side, I put this on the floor of the tank and buried it with gravel and put 2 large lava rocks at the entrance in the shape of a large letter "A". Make sure the canopy of the tank is very secure with no escape routes due to their remarkable escape skills. Lots of plants a must along with heavy filtration.
Tank mates include Puffers, mollies, goby's, mollies, etc...
Feeding them is a different story all together. I tried a variety of foods and feeding techniques and have learned that they will eat when good and ready. It is nothing for them to not eat for days, even weeks at a time. When you first get your eel, it probably won't eat for a week or so, don't worry, this is normal behavior. The eating habits are that of a snake. They do not need to be fed every day. Once or twice a week is sufficient. They do prefer live food so weening them off is a must, try waving around bits of food on a string in front of their lair opening, they will be fooled into thinking it is live but will learn otherwise in time. They will eventually accept dead food. Variety is the key.
Different foods are as follows
Cooked cocktail shrimp
live feeder fish (as they get larger)
What I've found they love the most is whole mussels.
Buy them at the supermarket for like .99 cents a pound and freeze them in the shell. A couple times a week, take one out and thaw it in tank water, open the shell and put the whole thing in front of the lair. This is very nutritous and provides a meal that they would normally find in their own environment. Don't waste your time on flakes, pellets or anything of the likes, it won't work.
All information is based on my personal experience. Not all may work for you. Hope this info helps.