We have RO/DI water stored for about a week. During that time, it is heated and aerated with a big powerhead in a covered plastic trashcan. We salt the water the day before water changes and use it within 24 hours. We've noticed salted water kept longer than 36 hours to begin leaving a whitish ring around the water line, which I believe is something precipitating out of the saltwater.
Heating and aerating while we're waiting for the next water change gives us the opportunity to salt it and use it right away in case we need to do an emergency water change. Got to admit we haven't have many such emergencies, but having the water heated to tank temps and fully aerated has been a blessing when we have needed it.
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