from what I read, the best way to keep an octopus from escaping a home tank is to not put it there in the first place.
liveaquaria.com, unfortunately, is selling these. Even they admit that the level of care required to keep an octopus is such that it is only "recommended" for professional level setting like research institutes or large public aquariums. (of course this is just a "recommendation" meaning they will sell them to anybody who ponies up the money and the extremely high shipping cost. alas.)
For starters: Absolute minimum required size tank is 120 gal, larger is better. And they are extremely strong and all rock needs to be glued solidly down lest the octo pull it over onto itself and die. You cannot keep any other creatures in the tank with it that you do not intend for it to eat, because it will. Most importantly you cannot have it in such a setting where it might get startled, such as by you walking or moving too closely in its vicinity. This may cause it to release its ink cloud in panic. This will require an immediate almost 100 percent water change or the octopus will die. It requires dim light at most, meaning that you are not likely going to see much of it. The mention of double-hulling the tank to keep octo and heater totally away from each other was noted above. Add on the extremely short lifespan as Arakkis notes and you are talking about a monumental expense and effort, for a very low enjoyment factor, for a very short time.
i understand the fascination with octopi and share it. The first time I saw one of these dwarf/mini types I lusted to have it too. But I just have to urge you to withstand this temptation. I would hate to see these become some kind of a fad in the hobby. Above and beyond our own personal issues the last thing we need to be doing is puting any more stress on the populations of any sea creatures, or any wild creatures for that matter. If you've ever seen the figures on how many birds, for instance, die in capture or shipping to fulfil the demand in the hobby market you know how devastating to populations it can be. (Yes there are captive breeding programs but they never seem to make a dent in the wild-capture market.)
just my humble opinion, and your mileage may vary and all that.
