Is this a new behavior, or did it happen soon after it was in the tank? Do you know what species it is?
I'm realizing that something that's not mentioned yet in the thread is the temperature-- maybe it's a species that wants colder water, and it's going up to escape the heat?
As has been mentioned, most of the time when we hear about octos climbing out of the tank a lot (as opposed to just exploring) it's because there's something wrong with the water, but this seems much more extreme.
Like ob, I'm wondering how it keeps from drying out and gets enough oxygen without taking an occasional quick dip.
I'd say it's the temp, if he went in right after the new water went in the tank, then got back out after a while, it may have been because he liked that temperature water, but when the temperature changed, he got back out
just my two cents