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while it's possible to put a dwarf species in a 29, they don't really make good pets, since they tend to be nocturnal, shy, and short-lived. If you're planning from the start, a 55-75gal tank with a sump would be your best bet. We used to think a 29gal was OK for a single bandensis cuttle, but a few people said that as full adults they seemed cramped in a 29.
You can get some idea of the best possible dwarf experiences by looking at the threads about trapper and varys and their offspring, but note that their owners only got to see them in the middle of the night using red lighting... it's a much less interactive pet than a larger octo.
O. mercatoris are pygmy and nocturnal (Varys and Trapper were mercs.). O. filosus has been renamed O. hummelincki and is not a pygmy but is diurnal and has a shorter arm span than most others with even a smaller mantle size. Octane (my hummelincki) was much larger than any of the others reported but not much larger than the top of the range listed in Norman's Cephalopods a World Guide. He out grew the 39 (that was purchased as and thought to be a 49 until measurments were taken because he out grew it) but died before his 55 was fully ready. Even with a sump I was changing water twice a week. Additionally, hummelincki like to swim (many don't) and a longer tank should be considered.
They are smaller in mantle size than O. hummelincki but have longer arms. If any non-dwarf would work, this one might but the arm length would still make it cramped. If you decide to use a smaller than recommended tank, a sump should be manditory in addition to extra water changes and heavy filtration.
Another problem is actually getting the species you order. A lot of suppliers don't identify their species properly so you might or you might not get what you were told you are getting. So, it's best to plan for the worst case scenario and just go with the 55 minimum.