Personally I would setup a small tank just for the octopus. That is probably the coolest HH I've ever seen. If you do keep it you there some things to keep in mind. I'm no expert this is just what I have read about octopus. The most important thing is a closed sealed locked and latched vault for a top, they are able to release latches and lift tops. Second is that they like to be entertained when feeding, play tug of war or create puzzles for them to solve to get their supper. Most articles I've come across suggest putting their food in a closed jar, yes they are capable of opening the jar. That's pretty much the basics only thing left is to inform you that octopus don't live very long. Good luck, I reiterate that you should set up a species only tank. A ten gallon would likely do the trick for now (once it outgrows you then have a medical/quarantine tank) and I can't speak for everyone but I would love to see more pics of that little guy.