I think cuttlefish are cooler than octopus
. Heard they're the most intelligent of the molusks. Some octopus/cuttlefish are only like 6", so you don't need a large tank, just lots of sump space. I heard you don't need much lighting either, usually just a single NO 50/50 bulb or something similar. The only two thingsthey DO need is a super tough lid (shimek writes of a colleague using 50 lbs of diving weights to hold a lid shut on a 6" octopus!), and EXACT water conditions 24/7. They can squeeze through a hole the size of their beaks! So if you have a 10" octopus/cuttlefish, a space 2mm thick is an absolute escape route!
Oh, and that they have lives even shorter than hamsters. You'll have to buy a new "best friend" every two years.
I think the water parameters could be easy to maintain with a well planned, high tech system like the one above. The low lighting means you don't have any algae screwing around with your pH, and you could probably just run a good fluidized bed filter, followed by an excellent nitrate reactor.
I read of one person's octopus climbing DOWN their 1/2" sump piping, and getting torn climbing inside a pump inside the sump.