Mixing New Salt with Old Salt??

[dcw1sfu]

Ok I do about a 15 gallon water change every 1.5 to 2 weeks. In order to do this I end up mixing 20 gallons of fresh salt water. Im currently using DD H2Ocean salt. I have to mix the extra 5ish gallons in order for my pump to be fully submerged in my Brute Salt water mixing barrel. Therefore ever time I make up new salt I end up dumping that 5 gallons out.

My question is do I need to dump that 2 week salt water out?? Can I just dump 15 gallons of fresh RODI into it and add the salt? Or is this going to cause issues with precipitate and the fact that 5 gallons of the salt mix is old water??