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To acclimate an octopus, the best thing to do is to very gently open the box when it arrives and let some light in.
Then untie the bag and over an hour drip water through an airline (knotted to slow down the flow) from the tank into the bag watching to make sure the octo doesn't climb out. (You can also gently pour the water with octo into a large bowl and drip into that). Once an hour has passed and the water is fairly diluted with new water, scoop the octo out and gently pour it into the new tank using a large glass or even your hand.
Leave the lights off the first day. Some octos are hungry right away and want a snack, so you might offer something.
I have found too, after dripping the octo still in the bag, that if I gently place the bag in the tank, the octo eventually comes out on his own. Although, Ink on arrival was sooo stressed that after dripping for an hour, she wouldn't come off the plastic, so what I did was cut away till there was only a small amount of plastic with Ink still attached. She eventually came off on her own, but she was sooo stressed, that it took her a good 12 hours to regroup enough to go and explore her new home. She was the only one out of all I've had where I've had to stretch the acclimation period because of her condition on arrival.