Octos are very sensitive to water quality. They need to be kept in a well established tank by themselves. Any crabs, snails, clams, shrimp or smaller fish you put in with them will be eaten. If you put them in with a shark then the shark will eat the octo...
Octos also need a very very tight fitting lid on their tank as they can get out of any opening larger than their beak (the only "bone" in their body) and are very good at "climbing" the tank glass and finding a way out.
Last bad thing is that, even under the best of conditions, they don't live long. Not the hobbyist's fault; most octo species entire life cycle is only around 1 year.
They are quite cool to look at though.