The Neptune Trident Marine Aquarium Water Analyzer is an add on to the Apex system that allows you to automate testing for levels of Calcium, Alkalinity, or Magnesium in your saltwater aquarium. It can also be paired with the Neptune DOS, which is sold separately, to create an automatic testing and dosing cycle to help maintain proper and consistent levels. Testing is also made much quieter with this unit, when placed inside a cabinet it is virtually silent. And because it is integrated with your Apex unit, you can also program alerts for out of range parameters and make adjustments on the app as well as view your tank stats.