The Aquariums Salinity Refractometer is a great way to keep your fish healthy and happy. This refractometer features two scales that can be used separately or together. The first scale is a hydrometer scale that can be used to measure the salt content in your water. The second scale is a parts per thousand scale that can be used to measure the amount of salt in your water. With its easy-to-use design, this refractometer is perfect for any fishkeeper.