I think a 90 is fine for skeletor, a good looking and nice eel to keep. The key is get him on the smaller side, train him to eat from a stick or tongs before introducing him into the display. They normally won't eat your fish, and may take a crab or shrimp. You have to use common sense, an adult eel right from the ocean and a clown goby won't make a good match. You will have many safe options in a 90g that won't be threatened by a skeletor.