If you don't mind paying for it, ***** has something called Real Ocean Water. The name of the company that makes it, well collects it, is Catalina Water Company.
www.catalinawater.com Check em out. Its real ocean water from the Pacific. They collect it, and filter it. I have been using this from the beginning, which is not too long ago, but I have have had zero water issues. PH is perfect, gravity is perfect. salinity is perfect, ect.... and no mixing!!!! It cost 9.99 for 5 gallons, which can get expensive quick if you have a large tank. I have a smaller tank, so its perfect for me.
Good luck,
Tom