A 55 would be fine for a small snow flake eel that was no bigger than 24 inches. But a 75 would be a better choice. Yes eels do tend to not move around much but if they do decide to the eel will be verry cramped in a 55 not much room to move once you consider they can get 3 feet long. And im shure you will have live rock in the tank that takes up alot of space.