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Well. it sounds like your tank is going through a fairly large ammonia spike. A partial water change would be a very good idea, as well as putting fresh carbon into your filter (make sure it is saltwater use carbon, without the white granules).
Contact your local lfs and ask if they will sell or give you some saltwater from their INVERT tanks...do not get any from a fish tank (copper,etc), or purchase the salt and mix some up.
Well, it is hard to say...it really depends on the tank and the octopus...some of them last through thick and thin, others give up the ghost quite easily.
Never is fun having a tank crash like that...all you can do is be prepared for the next time, by having supplies on hand...I would recommend calling your lfs and asking them if they can sell you water, that will speed up the water change aspect, as you won't have to wait for the salt to dissolve, etc...