The correct plural of “octopus” is “octopuses”.
People often write “octopi” instead because they assume that the plural noun is formed in the same way as Latin loanwords such as “fungus/fungi”. But “octopus” actually comes from Greek, where its original plural is “octopodes”. In English, it instead has the regular plural form “octopuses”.
For example, you might write “There are four octopuses in the aquarium.”