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Aquarium Hydrometers & Refractometers Useful Links: Hydrometers & Refractometers Related Products: Aquarium Salt Aquarium Hydrometer/Thermometers Hydrometers and thermometers combined in one unit from Red Sea and More. Aquarium Hydrometers Aquarium hydrometers from Aquarium Systems (Instant Ocean), Coralife (Deep Six), More and Red Sea. Aquarium Refractometers For the most accurate salinity readings use a refractometer. They are more expensive than a hydrometer, but they are more accurate and they will last much longer.