Good morning Tom.
I keep my mega crop in sealed containers.
The ziplock never works.
I mix 300 litres at a time.
This can take upto a week to use.
My water is saurced from the river no chlorine.
I use a Mosquito Dunk in each tank to get rid of any larvae.
After setting ph it will usually stay in range for 2-3 days.
But I check it daily before I feed.
I've been doing this for 2 summers now and no problem has occurred.
Just keep an eye on the ph and you can use it for a few days.
Happy Growing.