As awesome a pet I can imagine a chambered nautilus might be,they don't have a very good track record of living very long in aquariums,probably due to where they live and how they are collected and brought in to the hobby.Cephalopods in captivity have really mixed results. Nautilus, for example, lay down stress shell and then die prematurely. Octopi are dependent on species. In the case of pacific reds, you're talking about a potential to be rather large. Many octopi are celebrated escape artists and require very specific equipment like a locked down and sealed tank.