They move very quickly. They tend to quickly kill themselves by bumping their heads into aquarium walls.
If they don’t die from bleeding, trauma, or infection, or stress, they tend to die pretty quickly from asphyxiation. As whenever they bump into the wall, they get suprised, and spray ink. In an aquarium, the ink stagnates and blocks them from breathing properly.
World's crulest animal puts other animals in captivity and then calls them dumb for showing their natural behaviour in a completely alien envirment. More at 11.