I too, also have a 18" Fire Eel with a 12" Ornate and a 6" Del. The Ornate and the Eel have been housed together for about 16 months, and the Del's been in there since 3", with no problems what so ever. Although the Del seems to treat the Ornate as his "mom". He always seems to stay close. As a previous posted stated they all live peacefully, but you never know what might happen. I can only speak for my mine but I think that from what I've seen you should be fine. Just make sure the size differential isn't to much.