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Have you read all the octopus care articles? I don't actually have an octo, but because of the heat wave there has been a lot of discussion about keeping the tank cool. You can also go back and check old threads and you'll find other posts with the same questions and answers from experienced ceph keepers.
I have an octopus and have never needed a chiller until a few weeks ago but I simply use ice, a fan and if it gets to hot I keep the lights off. You should be fine the only problems I have been having are with my 90g it was about 87. but that tank has nothing in it. maybe looking into getting a biareus octopus (they can take a bit warmer temps.)