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There are a few different species of octopus that can be kept in a 55 gallon aquarium. One of the most common being the Briareus. From experince I find that briareus will change color more often and be a little more forgiving whan it comes to water quality but briareus are much more shy than bimacs. There are plently of other octopus that would do well in a 55 gallon aquarium but they are often hard to find and mostly obtained by mistake of a faulty labeling.your best bet would either to get a wild caught briareus or get a captive bred Bimac (If they become available) other than that getting a specific species of octopus is a gamble.