A hydrometer is a necessary accessory for saltwater aquariums. With a hydrometer you measure the salinity or density of the water. For the saltwater aquarist, measuring salinity or water density is important because marine fish are used to a constant salinity and do not tolerate fluctuations well.

 

The oldest type of hydrometer is the floating hydrometer. The hydrometer is placed in the water where it floats along the water surface, measuring the salinity. Often the cheapest version of a hydrometer, but not necessarily the best solution aesthetically. Also, the measurement becomes less accurate over time.

A needle hydrometer functions ass outside of the tank. It is a transparent container with a needle that is filled with the aquarium water. The needle indicates the mark of the corresponding salinity. This type of hydrometer is easy to use. After filling the container, you have to be careful that there are no air bubbles inside that could falsify the measurement. Like the floating hydrometer, the needle hydrometer loses accuracy over time.

A digital hydrometer (conduction hydrometer) measures the conductivity of the water and uses it to determine the salinity, which is shown on a digital display. With some models the probe remains permanently in the water, with others it is only inserted for testing. This is the most accurate, but also the most expensive type of hydrometer. This type of hydrometer also lasts much longer than simpler ones.

Another tool for measuring salinity is a refractometer. Instead of measuring the density of the water or the conductivity, the refractometer reads the salinity through optical properties of the water sample. This tool is about equal to the digital hydrometer in accuracy as well as price and longevity.