I would recommend 55 gallon aquarium as a minimum size. Hummelincki is the smallest (length wise, aculeatus has a smaller mantle and is more delicately built with considerably longer arms) of the common aquarium kept non-dwarfs (somewhere in the scarce info on this species I read that at one time it was classified as dwarf but then reclassified) however, Octane had over an 18" reach (sitting on the bottom he could reach the cover of a 2' tank) and at least a 3" head and mantle length. He out grew his 35 gallon (thought to be a 45 gallon until measurements were taken and translated) and we were in the process of completing a 55 for him when he died. The too small tank may have been the reason he started trying to climb out of his tank after 7 months in captivity. Serendipity, our female, was large dwarf size when she started brooding (never to be seen again). OhToo, our second male, is somewhere in between. He was recently transferred to the 55 from the 35 and makes use of the entire tank.