jeremy36, Young man, i would have to say that you have a load of problems happening at this time, for one you are no idea of the species you have that is already at 3' and it could be you were donated a Tessellata Moray in which case its no were near its adult stage as yet. If you can up load a photo, it would help you a great deal to learn just were you stand for either you adopted a peaceful moray, or a moray that will require a larger tank system and be all by itself.
Also, of the majority of marine eel species, a variety of them grow to a maximum of 2' and their happens to be greater then 1,000 species with any number of species unknown as yet. And as well, you were suggested a 3' eel to have a 120 with 180 better, i would go with the 180, also larger depending on if you be able to house as well any large groupers or so, for the tank size suggested not mean you can cage a large marine species in so close quarters and also add to its confines other marine species.
You see, if the eel you have is a Tessellata, you would require for it a far larger aquarium system and if it is a Tessellata, the system would be not less then 2500 gals. A Tessellata requires a great deal of open space to move about, they happen to be one type of eel that moves in the opening day or night, but not so much during their younger juvenile stages.
Take a look in search to see a Tessellata and know that the adult sizes you most see, their spots are larger and fewer. And if it be a Tessellata, you will have to have some serious thinking to do and you best have the money now to start a large system.
Buddy
PS. It may even be a spotted moray, for many also gotten this eel often enough.