arent eels aggressive?
Depends on what you mean by aggressive. Will they attack fish? No. Will they eat smaller fish? Yes.
Tanganyikan eels do great in a Tanganyikan tank. They are easy going, active, and don't bother anyone. I have 1 in a 29 gallon (he's still small). There are baby endlers in there that he doesn't touch. They have really small mouths so they can't really eat anything larger than fry. I guess once full size one could eat something like shellies, but maybe just young ones.