An ex-work mate of mine is into saltwater aquariums in a big way. He is also an avid scuba diver. One day whilst scuba diving off the coast of Sydney he saw a bright looking octopus. He liked it so much he took it home and put it into his salt water aquarium with his other fish he bought. A few days later he realized there was a few fish missing, then a week later 5 were gone. He realized the octopus was eating his prize fish. The fish knew, if the octopus was at one end of the tank the fish where at the other end. Eventually when all his fish has been consumed he tried to catch the now bloated octopus, but it would jam itself into a corner and he couldn’t get it out no matter how hard he tried. Eventually in frustration my workmate drained the tank to starve the octopus of oxygen. He then took it out of the tank and put it back in the ocean whence he caught it. Moral of the story – Don’t catch fish (or in this case an invertebrate) out of the wild to stock your fish tanks.