IMO you can keep mixed up salt water for up to 2 weeks as long as it is being constantly mixed with a powerhead and heated and your container is covered to prevent crap getting in and evaporation. I mean really - think about it - after it goes into our aquariums what is it other than heated and mixed salt water. A lot of people use the rubbermaid containers as well as the rubber livestock water tubs for sumps and fuges - again filled with mixed and heated salt water.
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