Octopuses will bite off their tentacles and die if they're stressed by bad water quality!

A stressed octopus will suffer from autophagy, or self-cannibalism. When deeply affected by stress, the octopus defenses are down. It's believed that this leaves them open for infection by a virus or bacteria.
The infection will make the octopus' arm irritated. They bite the tentacle off in hopes of regrowing a healthy one. However, if they're in bad water quality, they can't shake off the infection. Eventually the infected octopus dies with it's arms in tatters.
(Source)