In 'The Reef Aquarium vol 3', page 72, published 2005, Sprung and Delbeek write: "Tip: There is a nifty solution that you local retailer may be willing to help you with to eradicate all mantis shrimp during the quarantine and seeding of the rocks. Add a hungry octopus to the live rock holding tank, and the result is lots of shrimp and crab shells accumulating on the bottom of the tank (R.Rull, pers. comm.). Then remove the octopus and return it to the retailer. This technique is meant for new set-ups only, before the fish are introduced, because octopus may feed on fish. Caution: The aquarium or vat must have a secure lid when the octopus is inside, to keep it from escaping. There is also a risk of the octopus going over the overflow and getting into the drain." Its amazing how these kinds of bad ideas persist.