IMO the problem with many of the Echidna species eels is their girth as adults (maybe skeletor or polyzona or another small species might be best). As mentioned, they will cause breakage, rockslides, etc in a packed reef, IMO. Ribbon eels are great, but not small-fish safe. I would think snake eels (Myrichthys) would make a really great choice, since they don't get thick like Echidna eels, and don't eat fish (at least in my experience with one). But they will eat shrimp, cleaner or otherwise.
