I hate to play devils advocate at the risk of being "shunned", but sammy the ink from an octopus is harmful to the animal if it is not removed. I have been cycling a 20 gal for about 1 1/2 months now specifically for the use of keeping an atlantic brown octopus. So while the tank has been cycling i have been doing 1 1/2 months of research. Im not sure which issue but i think it was aquarium usa magazine did an entire article on keeping octopoda. And it does state that in a confined area like an aquarium the ink can be harmful to the animal. Now maybe I am wrong, but other resources that I have read all say the same thing.
As far as keeping one, know the facts:
1.) any fish kept with it, depending on size will either kill or be killed by the octopus
2.) just as stated above they will escape the smallest crack at the top whether it be where the filter is, or even pushing up the tank lid. Make sure the tank is sealed!
3.) they will rearange rock work. Make sure to place rock so that it wont break anything if it falls.
4.) have plenty of rocks. An octopus needs places to hide.
5.) make sure you have a really good skimmer. Water quality must be perfect.
6.) make sure you know what to feed it and how. The most common misused food is gold fish they do not get the nutrition they need from them. Among other things they like shrimp, SW fish, and crayfish. And as far as how to feed them, do it carefully if your finger is in the tank he can bite it, and it is said to be very painful.
7.) do not keep them with other octopus.
8.) find a young specimen their life span is only about 2 years.
Those are just some of the basics, and I know i left things out because it's 1 o' clock in the morning. Your best bet is research, research, and more research. Oh yeah and don't tap on the glass, it's like someone knocking on your door just to see how you'll react. It gets annoying after while.