When selecting the right instrument to measure the salinity in your aquarium, there are a handful of options available, but not all of them are created equally. For starters, there are hydrometers and refractometers. Hydrometers are inexpensive, and quick and easy to use. However, they can also be quite inaccurate and may give false readings if the swing arm sticks. Refractometers are far more accurate, but there are different models that either can or can’t compensate for temperature, and they can be quite expensive. On top of that, they have to be calibrated. If you have a refractometer that doesn’t compensate for temperature, or if it is calibrated incorrectly, it can be far worse than most hydrometers. We highly recommend using refractometers, but don’t skimp on the price and get the cheapest one you can find. Buy from a trusted brand and make sure to pony up for the calibration fluid.