it eats crabs, fish, snails, etc, it basically lives alone in the tank. well if he happens to bite you you sorta die in 15 minutes. its more deadly than any land animal. the octo has the same poison as box jellyfish, and if you know about those then its not good if you get stung. and the funny thin gis you dont even feel the bite until you lose your vision a couple seconds later. read this:
http://www.earlham.edu/~sheedjo/blue-ringedoctopus.htm
not so fun.