keeping of conditioned water........

Hey guys and gals, I had some questions about keeping of conditioned water. Figured I would start a thread in case anyone had some good info to share and to make the topic easier to search. I got my mom into keeping bettas and found her two really nice males. I set hey up with 2, 1 gal tanks with under gravel filtration and some live plants. I want to provide her with pre aged and conditioned water to do her water changes with and wasn't sure what kind of shelf life pre- mixed water has. I use Tetra Aqua safe and treat 5 gallons at a time. I also have other water conditioners such as betta fix, maroxy, and salts. Should these be mixed as needed only? Or can I provider her with pre mixed? Thanks