+1 to the lifespan issue. Octopi are very intelligent animals. They can solve basic problems to achieve goals of self-preservation, communicate with each other, and recognize people when in captivity. The only reason they aren't held to the same standard of animal intelligence as things like dolphins and chimpanzees is that they only live two or three years in the wild. And in captivity almost any animals lifespan is cut short. Between that and the fact that they are expensive, I'd advise against it.
If you do decide to establish a tank, it should be very large and for a single specimen. Also, octopus ink, while an interesting defense mechanism, can pollute a limited system like a small or mid-size aquarium. If you startle it, and it inks, you basically have to do a water change immediately.