If you get a dwarf octopus the they'd be fine in a nano aquarium (20 gallons at least though) and can be kept in groups. I kept one a for a while but didn't see it too often. Keep lots of rockwork in the aquarium so they feel safer and will come out more. They are nocturnal so don't expect them to come out during the day. Larger octopuses like the bimac need at least 55g I believe I read somewhere but obviously a 75g is a much better idea. If you're going to house any type of octopus you need to make sure there is no possible way they can exit the tank or get inside a powerhead or filter. If I remember right, they can fit into a hole the size of their beak, which is pretty small. Also no fish or invert you wish to keep alive should be in the same aquarium as the octopus. I fed mine live crab, snails, and hermits. I don't know if they accept frozen food or not.
That's all I remember, but I did most of my research on that ceph forum mentioned earlier.