How to store Soda Water or other Home Made Sodas?
More sodastream bottles is the best option if you are starting from a sodastream.

The best other alternative is to act just like a really old-fashioned soda counter. 


Mix and store your syrups/flavors.
Measure syrup into the glass.
Add plain soda-water and mix.


Thus, your sodastream bottle only ever has plain carbonated water in it, not a specific flavor.