25% off MarineDepot.com Refractometers - Sale ends midnight 1/16/12 Refractometers measure salinity based on light refraction through your tank water. They are easy-to-read and provide consistently accurate results with minimal maintenance. For ease of use and reliable results, refractometers are your best option for any marine tank. Marine Depot Aquarium Refractometer - Regularly $49.99, Now $37.49 A refractometer is a must-have testing tool for all saltwater aquarium hobbyists. The proper salinity is crucial for the survival and health of your fish, coral and inverts. This high-quality refractometer is extremely accurate, easy-to-use and calibrate. Calibration is done with just a few drops of distilled water. This model also features ATC (automatic temperature compensation). Marine Depot Aquarium Pro Refractometer - Regularly $64.99, Now $48.74 Specifically designed for aquarium use for measuring the salinity of Natural Sea Water. Almost all handheld saltwater refractometers available in the hobby are designed and calibrated for testing of brine solution, i.e. sodium chloride, and are not calibrated for the refractive index of NSW. This is a very important distinction. Other salinity refractometers used for marine aquariums are designed to test saltwater—nothing else! The Marine Depot Pro Refractometer is designed to measure the salinity of NSW and takes into account all the other elements that comprise salinity in the marine water sample.