Bubble argon through the water for 1-2 hours, removing all the dissolved oxygen.
Where does the oxygen go? Argon is inert right? So it’s not binding with it or anything like that?
Another gas bubbling through the water "knocks" the dissolved gasses out of the water. So the oxygen bubbles up out of the water, along with the argon.
Presumably, the dissolved oxygen is displaced into the atmosphere.
De-oxygenate the water. In the same manner that you can suffocate a person by removing the oxygen from the air(it's not like the only way to suffocate is in a vacuum).