Density is measured with a hydrometer, whose buoyancy increases as the water
gets saltier. In the aquarium trade, most hydrometers also include a thermometer.
The specific gravity at water level must be read with care; in fact, it is preferable
to use the hydrometer outside the aquarium, as the movement of the water makes it
difficult to read. In this case, decant the water into a test tube or a transparent
container (a PVC bottle, for example) and float the hydrometer in it. When it stops
moving, read the value corresponding to the level of the water (1.023 in the diagram
below, and not 1.022). To check whether your hydrometer is working properly, just
measure the density of a distilled or very soft water: it must equal 1.000.
The hydrometer, an indispensable tool in marine aquariums.