I don't even know why they sell hydrometers. What is the point? All they do is cause havock. I have 1 that is 5 points off. My salinity is at 1.025, but my hydrometer sais 1.020. I always compensate. Take your water to a fish dealer and have them measure the salinity. Then see how many points off your hydrometer is, then compensate for your own tank.