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Welcome to one of the down sides of octopus keeping. Lack of knowledge about keeping octopuses runs high and you are one of the unfortunate ones put in an unrecoverable situation. You will need to do 50% water change daily just to try to keep enough O2 in the tank since your tank is already showing it can't handle anything close to the waste.
Please don't risk putting him back in the ocean. Dave's mention of euthanasia does not come lightly. With the water parameter issues, returning him to the LFS is likely the best option.
You should contact you LFS and find out if they will take it back. Or you could post it for sale, members here, as D mentioned, are always looking. But the sooner the better. It won't last long in it's current situation. It's hard to learn a lesson the hard way. Unfortunately this happens. I am sorry this happened you and the octopus but you can help it find a better home.
Do you have a skimmer? I worry it is not getting enough oxygen in your tank.