Types of Hydrometers

A hydrometer is an essential piece of aquarium equipment for fishkeepers. A hydrometer is a device that measures salinity or specific gravity of water. This is a technical way of saying "how much salt is in the water." This information is important for saltwater hobbyists as marine fish are used to a very constant salinity and do not react well to changes in salt levels. At the same time, advanced freshwater aquarists may find hydrometers useful for raising brine shrimp as a fish food, or preparing tanks for brackish water fish.

  1. Floating Hydrometer

    • This is the oldest type of hydrometer. Small markings on the sides correspond to the salinity of the tank. To read the tank's salinity, this type of hydrometer is placed inside the aquarium. The hydrometer will balance itself with the waterline at the marker for the tank's salinity. This is often the cheapest type of hydrometer. However, it is aesthetically unpleasing and some larger fish may try to eat it. It also loses accuracy over time.

    Needle Hydrometer

    • This type of hydrometer operates outside of the aquarium. It is a clear box with a large plastic "needle" inside, and a graduated series of marked salinities. The box is filled with water to a waterline and the needle will float until it points to the salinity. It is slightly more expensive than the floating hydrometer, but it is easier to read and use. When using this type of hydrometer, you should tap the box after filling it to dislodge any air bubbles that could be altering the readings. Like the floating hydrometer, it becomes less accurate with time.

    Conduction Hydrometer

    • This is the most recent version of the hydrometer. It uses a conductivity probe to measure the water's salinity. In some models, the probe is left in the tank continuously and in others it is only placed in the aquarium when testing the water. All versions have a digital readout. This is the most accurate and easy-to-use hydrometer, but it is also the most expensive, often costing more than a hundred dollars while other types of hydrometers are usually under fifteen dollars. However, conductivity hydrometers often last longer than more primitive hydrometers, which become less accurate over time.

    Refractometer

    • A refractometer is another tool for measuring salinity. Instead of measuring the density of water directly like needle or floating hydrometers, or by conductivity like the digital hydrometers, it measures the salinity through the water sample's optical properties. It is more often seen in science labs than aquarium supply stores, but it is comparable to a digital hydrometer in accuracy and ease of use. Unfortunately, it is also comparable in price to a digital hydrometer. It is also more durable than floating or needle hydrometers.