The dwarfs can be kept in a 20 gallon. they are one species that can be kept in multiples. It depends on what our expectations are. They are very nocturnal and will only emerge in complete darkness. Octopuses cannot see red light. A common practice is to put a red light on the tank. You can leave it on 24/7 or have it come on when tank light turns off. You will never see them otherwise. I personally prefer the larger species. When I kept dwarfs, all I ever saw was an occasional arm grab food. Good luck, hope it works out for you.
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