Tough to make a choice with the obvious trade offs. The zebra is a crustacean eater but as you noted too big for your tank. The Gymnothrax morays are common but will eat fish, except one you might conisder: the dwarf golden moray. It would eat fish small enough for it to fit in its mouth, but it reaches a maximum of about 7-9" long, so its too small to eat most fish. I bought one from J&L and it is in a 112g with fish and shrimp and does not bother the shrimp or even smaller fish in my system - including a clown goby, and a barnacle blenny. Like other morays its eyesight is terrible but its sense of small outstanding. I think the fact that i keep it fed decrease the chance that it will go after the cleaner shrimp or small fish. The only donwside to these guys is thae are hard to find and expensive, but that is relative (was worth it to me to get one), and they are reclusive, but very cool. Wolf eels are interesting too, but are elongated gobies in reality.
