Generally, you want to keep most of your storage water as RO/DI, mainly for the ease of storage. RO/DI water is inert, meaning less wear-and-tear on the container and any equipment. Also, in case of an emergency RO/DI can be used for any water needs while premixed saltwater is good only for your aquarium. It definitely doesn't hurt to have enough saltwater for a worst-case scenario, but keeping vast reserves isn't necessary.
With premixed saltwater, you're only limited by your ability to keep the salt evenly mixed in the water over time. In a container, salt mix can slowly precipitate back out of the water and settle to the bottom. As the salt settles out, lower salinity water sinks and lower salinity water rises, creating an imbalance in salinity throughout the container.
In the shop, premixed saltwater in the big vat stirs 24/7 via a small powerhead to prevent this. In a smaller container, like a bucket or a deltang jug, you could simply use a powerhead to stir the water thoroughly before adding it to your system.
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