Buying ready mixed water

[Nippy]

I buy ready mixed salt water from my shop.

I do my water change straight away, but always have some left, does the water i have left have a shelf life - can I use whats left to do the next water change in 2 weeks?
It's kept in a plastic water holder.

 

[reefman5511]

You can use it for your next water change. Just keep the container sealed so you don't lose any water from evaporation.

 

[The Kapenta Kid]

I'd also add keep it in the dark. Otherwise algae may grow in the container.