The ESV B-Ionic Seawater System is a premium four-part system designed to mix synthetic seawater for saltwater reef aquariums. It offers the purest and most consistent water by using ultra-low heavy metal sodium chloride and USP grade magnesium sulfate, ensuring an environment that supports healthy coral and marine life growth. This refill unit makes 200 gallons of seawater and avoids chemical variations and humidity contamination issues typically found in dry blends.