I've heard of plenty of people who have a phobia of their partner's pet snakes or spiders, etc. I suppose it would be a bigger deal when it's an aquarium because a snake or spider vivarium isn't necessarily going to be on full display and isn't normally considered a relaxing thing to watch. But if your eel isn't always in view you can just tell him when he can look, I suppose. Not ideal, but unless he's willing to work on overcoming his fear, there isn't much that can be done about it.