These are often sold as "freshwater" morays, because I believe they are born in and captured in fresh water. However, they actually require brackish water as they get older, and can be kept in a full marine environment when adults, but it's not required to go that high in salinity. If you want to keep it in its own tank, I would think a 120 or 125 would suffice. What you will want to do is setup the separate tank as full freshwater, and then gradually increase the salinity with your water changes over the next few months until you reach the point where you want to be. I would recommend getting to at least 1.015sg for these guys, but you can go to a full marine salinity of 1.023-1.025 if you want to. Going to full marine will allow you to add saltwater fish, and use all of the marine gadgets, such as protein skimmers.
Hopefully some others can get you some more in depth information, but I think you can use the above for a good start.
centimeters long.