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50 Gallon would do...at adult size they reach a mantle length of 12 cm. (4.5 in.) and a tentacle length of 60 cm. (2 ft.) with a total of approx. 2 1/2 ft. I believe their tropical because their native to California coasts and Florida. They are known especially for their long tentacles and known as Carribean Reef Octopuses.
Toms Carribean had 11 of them in stock last week...but just got updated today and now only 1 is in stock unless its already taken which I assume. He sold 9 of them today in less than 5 hours. Good luck.