blue ring octopuses are very poisonus, and the only one that is small enough to survive in that small ov tank. most of the normal octopuses need at least a 30(my book says that the minimum cap is 55, with the tank dedicated to the octopus). I would suggest like a 60+ with mabey like a lionfish and and an eel and an octopus.
Farewell,
Eelman