I have owned 2 octopi.. here is my advise....
1. THey will more than likely eat any small fish and shrimp, crabs, snails, etc. in your tank.
2. They will not bother larger fish as much, but can be picked on by triggers.
3. THey are nocturnal, so if you want to really see them, I suggest getting a red night light for the tank.. they cant see that spectrum.
4. Ideally, put them in their own tank.
5. Feed them small live crabs if you can.. IE,... the ones you find at the beach under rocks.
6. Throw in 2 damsels. They will add some action to the tank and if it ever is starving, it will eat one. I have had damsels in with mine in an eclipse system 12 for months and they dont even bother each other.
7. LOTS of hiding places.
8. A common myth is that if they ink they will die. Mine inked his tank the other day and without water change is just fine. Im not saying to not change the water after an inking, Just that an inking is not a death sentence.
People will say that they will kill all your fish, ink the tank, and die. When in reality, they are shy but very smart creatures. Whatever you do, seal your tank well!!!!
Hope this helps!