text: fish don't live bery long in water that has been boiled and brought back to room temperature. why do you think this is? boiling does not remove anything but gases dissolved in the water. minerals are actually concentrated by boiling since water is removed, but the dissolved solids remain. so, the correct answer is that the oxygen is driven off. oxygen has no significant solubility in water that hot.  if you really need to boil the water to kill bacteria or something, you can re-aerate the water with an aquarium air rock designed for this purpose. it releases zillions of tiny bubbles into the water. i'd suggest running it for a day or two for a 20 gal tank before reintroducing the fish.