They are hard to keep in the tank. Octopus are problem-solvers and very creative, and they can fit their bodies through the tiniest of gaps. So it is extremely hard to keep them from escaping a tank if they want to, harder than any eel i've heard of. So that is problem #1, keeping them in the tank in the first place.
As far as being reef safe goes, most octopus will knock off every fish in the tank, so theyre not fish-safe, that's for sure.
Also, common octopus in the aquarium hobby are unfortunately some of the most deadly, so that's a problem. It's not like getting tagged by an eel or lionfish, a blue-ring just kills you. A little risky for me.:nervous: