Actually, there are 3 cories a bunch of Otos and a couple pencilfish (usually come out after hours) with the eel. When we first introduced them, 1 or 2 disappeared, but they've been fine together for years since. I guess the eel isn't hungry enough to bother with them, it's usually fed well every to every other day. The worry would be the shock, which apparently dosen't bother them either.