Most folks keep octos by themselves but there are exceptions. 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.