Saltwater fish hobbyists measure the density of salt water to tell how much salt is in the water. Most marine waters are very stable in terms of salinity, so saltwater organisms require a very specific salinity to thrive. There are several ways to measure this critical water parameter.
The most common, and cheapest, way to measure water density is with specific gravity. Specific gravity is a ratio of a water's density to that of pure water. Tools called hydrometers measure water density. The cheapest hydrometers are box hydrometers. In these, water is added into a clear box. Inside, a plastic "needle" floats and points to the water's specific gravity. The floating hydrometer resembles a thermometer. These hydrometers float in the aquarium water, and lines within the hydrometer will indicate the salinity. These are more expensive, but more accurate than box hydrometers.
Measuring the water's refractive index is a more sophisticated way to measure water's density and salinity. This method uses a tool called a refractometer, which measures how much water bends light. This property changes based on the amount of salt dissolved in water. These instruments may require calibration yearly. Additionally, this method of measuring salinity is very temperature-dependent, though most refractometers are set up to measure water at tropical marine temperatures. Refractometers are much more expensive than hydrometers, but give more accurate results.
Conductivity is another method of measuring the salinity, and indirectly the density, of water. An electrical probe is inserted into the aquarium or a water sample, and the probe's electronics measure the water's ability to conduct electricity. The amount of salt ions dissolved in water changes the conductivity of water. These units can typically provide the salinity in several different units, including converting it to specific gravity and refractive index. They typically cost more than a hydrometer, but less than a refractometer.
However you measure water, most marine fish tanks need roughly the same salinity/density. The desired salinity is 35 ppt of salt. In terms of specific gravity, that comes out to 1.025. However, a range of 1.022 to 1.025 is acceptable for most marine fish. In terms of a refractometer, you will want to achieve a refractive index of 1.33940. Above all, you want to avoid sudden changes in salinity and density, so make sure the water parameters stay within these ranges.
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