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depmds on which species and what size you tank is... so far all attempts to keep two octos together in a home aquarium has failed with one eating the other and that was with bimacs who are supposed to be more tolerant of conspecifics
I've got 3 vulgaris in a tank around 2000 litres, they are all about 60cm arm span and they live fine in there. It's an unthemed tank with pipes which are moved around daily so that they cannot establish a territory and therefore a hirachy. They have been there for around 6 months now very happy. A lot of Aquaria keep Eldone cirrhosa in captive groups of around 5 animals and I've only seen very few peoblems.
Of the other octo species i'vce kept i've never had them together. Simply dont have the room for a couple of GPOs in a tank!
I wouldn't fancy keeping vulgaris in a much smaller tank with more than one of them, they are still quite fiesty little things. Could probably keep E. cirrhosa in a 150Gal Aquaria I presume... never seen them fight before. I think I had 6 at one point in a tank around 650 litres, i'll have to double check the volume of that one though.