Old thread, I know; but I figured I'd chime in. I agree with Tangster in that salt would have no shelf life. As for the trace elements, neither would they, I'd imagine. As long as these items don't migrate out somehow there would be no chemical difference over time. Of course, darkness and as airtight a seal as one could achieve can only help. As for anerobic bacteria, they have to eat--so if the water wasn't contaminated when prepared, not only should anerobic bacteria not be present, but if they somehow were; they should starve to death.
So long as it's thoroughly mixed and oxygenated before use, it should be as good as the day it was prepared.
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