Most octos will adapt to frozen fish or shrimp as a primary diet. That crab will never know what hit it. Octo's don't really like fish that much but they will take adult guppies or goldfish as a pleasant change from a steady diet of frozen fish or shrimp. You should plan on feeding your octo at least every other day. Octopuses have comparatively short lifespans.
Most tropical species live a year or less.
Cooler water species may stretch that out to 18-20 months after hatching. So it's no wonder that so many people buy an octo at the pet shop and it dies or lays eggs after a month or two. They've already gone through their normal lifecycle in nature by the time they are collected. The local aquarium shop never does. Heck, they never know what species they have.