octopi need excellent water and tight fitting secure lids, and often are very aggressive so most fish put in with them do not have a very good chances for survival(some are not this bad, but most are) or the fish would be big and aggressive enouhg tho turn the tables, but the odds are we can't keep both in for long
they also need good skimming and lots of rock wiht caves adn most like coarser substrates
use caution not to spook it, and approcah slowly, if spooked it will spray ink, which is often lethal to itself and many fish
other than that, they are very intelligent and can normally find a way out of most tanks(if there is any) all it needs is a small opening or a lid it can move(strong too), and oftne can be trained for hand feeding once it gets to know you
HTH