The biggest problem with mixing synthetic seawater is not storage, but that it hasn't had enough time when initially mixed to be able to fully develop the carbonate/bicarbonate buffering system, and for CO2 to contribute to this system at saturation. So long as the water is well-circulated, aereated, kept at aquaria temp and stored in an open non-reactive container (to maintain CO2 and O2 saturation with the atmosphere), there is little reason not to keep seawater in storage for periods of up to a week at a time. There may become an issue with microbiological growth after a week in a container where there are no filter feeders or other biological filtration processes occuring (speculation on my part, not a known fact). Biggest concern would be evaporation of surface water, as it would change the salinity of stored water. If you store it, check the salinity, temp, and pH and adjust any variations accordingly prior to using it in a system containing live creatures.BBB said:Can you age your new salt mix too long before using it?
my experience and opinion only, your milage may vary