Sea Salt Measurements in a Fish Tank

In the marine aquarium, salinity is life. Ocean fish in nature enjoy an extremely stable salt content. In captivity, these organisms do not tolerate much salinity deviation; it's partly why saltwater tanks are challenging. You have to take several steps to keep saltwater aquariums stable. Several means of measuring salinity exist.

  1. Ideal Levels

    • Your aquarium water should mimic the salinity or salt content of ocean water. Across different oceans, water typically has 35 parts per million of salt. At tropical temperatures -- and most aquarium fish come from the tropics -- this translates to a specific gravity of 1.025. Specific gravity is an indirect measure of the amount of salt in the water. It is a unitless measurement, since it is understood to be a comparison to the density of theoretically pure water. While ocean water has a specific gravity of 1.025, fish and marine invertebrates can comfortably live with specific gravity between 1.022 and 1.025.

    Refractometer

    • Refractometers measure the angle at which water bends light. This property changes based on the salinity of water. Refractometers provide precise and accurate measurements of the salt content of water. However, these are expensive pieces of equipment, and it takes practice to read them. Refractometers usually give their reads in parts per million or ppm.

    Hydrometer

    • Hydrometers cost less than refractometers and produce somewhat less accurate results. However, for the purposes of the saltwater hobbyist, the results of a hydrometer are sufficiently accurate. Pet stores that carry saltwater equipment typically sell two types of hydrometer: the older floating models and the newer floating-arm models. Floating hydrometers look like glass aquarium thermometers. They float upright on the water surface. Based on the density of the water, these hydrometers float either higher or lower on the water. Floating-arm hydrometers are plastic containers in which you pour a sample of the aquarium water; a plastic needle gauge inside triggered by the water's density will point at a value that indicates salinity. The latter type of hydrometer is easy to read and harder to break. In both types of hydrometer, you must make sure no bubbles are extant, throwing off readings.

    Adjusting Salinity

    • To ensure the proper salinity, you have to take certain steps. First, always make your saltwater with salt mix and RO water. Salt mix is a synthetic mixture of salt and other minerals sold by pet shops. RO water is water purified through reverse osmosis, a process that removes almost all minerals from water. Tap water usually contains chlorine, heavy metals and dissolved minerals, harmless to humans but potentially dangerous to marine life. The RO process removes these contaminants. Replace any water lost through evaporation with more RO water but no salt mix. Salt does not evaporate, so you do not have to replace it.