It's best to keep a single octopus by itself because any other living thing that's kept with it can and will be injured/maimed/killed/eaten in no time at all. You'll have to make sure that the top of the aquarium is perfectly sealed since an octopus can and will escape through an opening the size of its eye. They're very strong, so the cover will need to be weighted down sufficiently. As you can see, HOB filters and skimmers are a very poor choice for an octopus aquarium, and a sump is preferred, especially if your aquarium is drilled.
Think of an octupus as if it was an immensely strong small child: it wants to explore and check out anything and everything yet can also move relatively heavy objects in comparison to its body size.