We have a forum for that too
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Forums->Sources for Cephalopods and Food)
Your size question is typical for most people interested in becoming keepers and we need to address this in a common post. I think Nancy is working on a mini-article/post to present tank sizes for commonly available octopuses (hint, hint). The short of it (if I am ever short with a post) is that octopuses need a larger tank than you are planning.
There is one commonly collected dwarf (there are over 300 species and about half are in water depths - don't quote me, it may be somewhat less than half - that would make them viable for an aquarium but species availability is very small), O. mercatoris that "fits" a small tank (15-30 gallons) and is the only one we have successfully kept together in multiples. However, it is nocturnal and not often human interactive. I enjoy keeping the mercs but in addition to other species (I started with these, however, and still enjoy them).
Aquarium sized animals only live for somewhere between 12 and 18 month (natural life span) and we don't have anyone breeding the few that are viable to be captive bred (you will read about small and large egg species in Nancy's book) so setting up a tank needs to take into consideration that there will be multiple animals housed over time and that availbility of a specific species varies. A 45+ gallon tank will house a small hummelincki (but we see some variation in size and the larger ones do better in a larger tank), an aculeatus or member of the abdopus complex, a small macropus complex animal who's species is not clear (both Indonesian, primarily at LFS or through Live Aquaria) and a bimac (currently only available if you catch it yourself and needs a chiller - a 55+ may be more appropriate for this species).
The most commonly available animal is O. briareus because it is often found in crab/lobster traps and a few of the licensed Florida fish collectors will buy and sell them to the public during crab season. O. briareus and the larger hummelincki need 65+ gallon tanks.
Sumps are heavily recommended for both the extra water volume and for securing the top.