If you want to keep an octopus and have it enjoy its short lifespan, put it in a larger tank. It will be more likely to show you its dynamic camo abilities and problem solving skills, as well as get the exercise it should have. Sure, you can keep a Bimac in a 20, and you can live in a closet too. I bet you'd be happier and much healthier in a larger space though. See my point?
Cuttlefish S. Bandensis are good candidates for the smaller aquarium, 30 gallon minimum. They only reach 3-4 inches, and breed readily (if they don't eat each other). S. Officianalis grow fairly large and need a 150+. They tend to jet backwards when frightened, so putting them in a small tank would result in them most likely busting their mantles open, getting infections, and dieing, before their year and a half or so has expired.
I kept O. Mercatoris for 4 months, of its 6-8 month lifespan. It was not a baby when I got it. They are easy to maintain, don't require quite as stout of filtration, but only eat LIVE food (shrimp, etc.) They don't need a chiller, can be kept at lower 70's.
And I must say he enjoyed sharing a tank with other fish. When I put him alone in his own tank, he only stayed in his shell, never came out. If I were to guess, I would say he was bored. This behaviour changed as soon as I put him back in with the other fish. He was in a large critter keeper mind you. The dwarf lion would have eaten him, though he thought he could take it...
TONMO is a must for anyone looking for Cephalopod information.