Never kept either of the Bimac species. Kept the Caribbean two-spots (O. filosus aka O. hummelincki) though, often confused with Bimacs.
Really need a 55 gallon or larger for most of the commonly sold octopuses. The bigger the tank the better. My 75 gallon is pushing it for the O. briareus I have currently, arm-span is 30 inches, and it's not even the maximum potential size which would be about 48 inch arm-span, the length of your typical 55 gallon.
Honestly you could get away with O. mercatoris quite easily in a 29 gallon, but they are strictly nocturnal, only come out late at night and absolutely refuse to come out if there are any lights on. They don't exhibit the entertaining interactive behaviors that the larger species do, very shy species. On the plus side though, they are apparently a social species so you could keep many in a 29 gallon. They only grow to about 5 inches long max. A lot of people have had success raising them from eggs, so if you got a brooding female you could raise another generation after she passes. They only live 8 months or so on average, but if you kept several you could potentially keep several generations until inbreeding causes any significant problems.
If you kept up water changes regularly once a week, maybe twice a week if needed, and you had a really good skimmer, you might could get away with an Abdopus aculeatus. They aren't pygmies, but they don't get as big as most of the mid-sized common octopuses.
Something to consider is that octopuses like to explore and move around, and the more room you have, the more entertained they will be in the aquarium, and the more behaviors you will see them exhibit. I keep my Abdopus in a 60 gallon cube, and it has lots of room to swim and actually walk on the sand, something I never see my O. briareus do, because it's too big to swim in the 75 gallon, and there isn't enough open space for it to walk on the sand, not even sure if O. briareus does walk.
It's worth the patience and extra expense to set it up right the first time. You will be rewarded for your efforts. Don't get discouraged, just postpone your plans until you can provide the right home and find the right species.
PS. Looks like you already got some good advice on TONMO, which means you're doing your homework. Thank you for that.
