Did you know that distilled water weighs one kilogram per liter, while saltwater weighs between 1.024 and 1.029 kilograms per liter? Hydrometers are used to measure the density or salinity of the water. The density of the water is a translation of the strength of the saltwater, so a hydrometer is an essential tool in the maintenance of your marine (saltwater) aquarium.

There are two types of hydrometers: floating and swing needle. The swing needle is the more modern version - it will provide a specific gravity reading independent of the temperature of the water. A floating hydrometer is more expensive. However, you must calculate any temperature changes different from 75°F.

You must maintain a constant specific gravity reading. When you first set up your saltwater tank, you should read the hydrometer frequently. Once you have maintained a relatively constant specific gravity, you will need to read the hydrometer every two or three days.