Aquarium Refractometers

Aquarium refractometers and hydrometers measure the specific gravity (density) of water, which is determined by the amount of salt saturation. Refractometers are more accurate, so they are a good investment for the committed saltwater fish hobbyist. Keep your salinity constant by replacing evaporated water on a regular basis. (Do not use saltwater for top-offs, rather use it only for water changes - salt doesn't evaporate.)