How long can I store saltwater made with H2Ocean salt? | D-D The Aquarium Solution
How long can I store saltwater made with H2Ocean salt?
We suggest the salt mix is used within 24 hours of mixing as this will be when the solution will be at its best. However if you keep the containers sealed you should be able to keep the salt at least a couple of weeks.
The downside of storing the mixed saltwater especially in still containers is that CO2 can build up. With no agitation by pumps the CO2 cannot be blown off via gaseous exchange at the water surface. This will lead to a suppressed Ph and also some precipitation of calcium carbonate, some kits may read a slightly lower Alkalinity but this should increase a little again once the water is agitated well .
The other down side to storage maybe of bacterial nature, as we all know new aquariums need to cycle when set up as nitrifying bacteria start to increase in numbers . If you keep the water longer than 2 weeks bacterias can start to grow within the water and this can be an acidic process and can slowly start to reduce Alkalinity and also produce cloudy water. 
If you are going to be storing water we can suggest doing so by mixing the water well for the first 3-6 hours, then empty into a sealed container full to the brim with water to displace any air and store in a cool dark place as this will also help prevent any algae or cyanobacterias from starting form. Also use high quality RO water for the mix.
Generally you should not see a drop in Alk in stored water that is stored correctly as there wont be any biological or chemical changes to the water via external influence.