If you’re building a marine aquarium and the thought of putting an octopus in it crosses your mind, consider not doing that. Consider not doing that so hard that you put this ill-conceived notion to permanent rest, no matter how much fun Finding Dory made it sound. An octopus makes for a difficult, finicky, and potentially even dangerous marine-aquarium inhabitant, best left to nature and specialists.
Octopodes are solitary. They dislike the company of their own kind and will frequently kill other octopodes that cannot relocate to a greater distance, such as tankmates, outside of breeding periods. An octopus must nearly always be kept alone.
Octopodes are really smart. They are mentally agile carnivores used to spending their energy and intelligence ranging far and hunting live, resisting prey. They are smart enough to solve puzzles and open jars. They will eat almost any other creature kept with them, including fish and invertebrates, that isn’t big enough to eat them, and a number of things that are. They need toys to keep them entertained in a place as potentially boring as a home aquarium, or that boredom will affect their health and encourage them to get into trouble around your house. Yes, your house.
Octopodes are really strong. Most of an octopus’s body is muscle, and they can exert much more force than the average human imagines is possible. Professional enclosures that contain them have weights on the lids; some facilities have someone guarding the tank 24-7 to dissuade escape attempts. Even so, octopodes are smart enough to notice shift changes and to set up escape routes before they actually use them. An escaped octopus will investigate cupboards, locate its food and gorge itself, attack or otherwise terrify any free-roaming pets like cats, and otherwise make a big, wet nuisance of itself before, probably, returning to its tank on its own at some point.
Octopodes are venomous. At the center of the tentacles is a parrot-like beak with associated venom glands, usually used to sedate prey during consumption. The blue-ringed octopus, a tiny species that is occasionally seen in specialty stores because of its much more manageable size than most of its kin, has a bite that is lethal to humans within minutes. Other octopodes aren’t as dangerous, but they’re bigger.
Octopodes are basically liquid. They can pour themselves through any opening big enough for their beaks. A heavy lid is worth nothing if an octopus can slide or lift it even a handful of inches. Once the octopus is free, few barriers can restrain its movements thereafter. Still thinking about the venom?
Octopodes have short lifespans. They’re as tricky to deal with as the most challenging parrots, but live only 3-5 years when everything is going well, to the multiple decades a parrot might live.
So if you think you can handle a beefy venomous escape-artist aquaparrot that hates everyone and lives for less time than it takes to get good at taking care of it…go for it.