In truth, the two words don't have the same meaning, even though they are often used interchangeably. Acclimate means to adjust to a single environmental factor, where as acclimatize means to adapt to many. Acclimatize is generally used for adaptation of organisms to changing seasons in the wild, where as acclimate is generally used for changes in a laboratory conditions in which factors are controlled. I have had a professor or two that have been very uppity about the distinction. To tell you the truth, however, I don't know which is more appropriate for adjusting an octopus to a new tank.