Haha , a big one!? that would be cool , but you would need a lid that secures the top all the way around , there couldnt be a quater sized hole anywhere otherwise he would escape, its easy to do little ones , you can buy them from asian markets for 10 bucks LIVE. Then if you buy them there take them to a fish store on trade in they give you 50$ cash! idk how i know that haha add more details about the octopus size and tank if it can be closed off