About Salinity

Salinity is a measure of the total amount of dissolved salts in water. Seawater is not simply salt dissolved in water — it is a complex solution containing calcium, magnesium, potassium, and dozens of other trace elements that marine life depends on. In marine and brackish aquariums, salinity is one of the most critical parameters to monitor and maintain. The ocean averages between 34 and 37 ppt of dissolved salts.

How salinity is measured

Salinity can be measured two ways: directly in parts per thousand (ppt), or indirectly through specific gravity. Most hobbyists use specific gravity because it is easy to measure with a hydrometer or refractometer.

Specific gravity is the ratio of the density of your aquarium water to the density of pure water. Pure water has a specific gravity of exactly 1.000. Saltwater is denser than pure water, so its specific gravity is higher — the more dissolved salt, the higher the reading. The specific gravity of natural seawater at 35 ppt is 1.026.

Tools for measuring salinity

Three tools are commonly used: a swing-arm hydrometer, a refractometer, and a digital refractometer. Swing-arm hydrometers are the most affordable but least precise, refractometers are more accurate and better suited for reef tanks, and digital refractometers are the most precise and easiest to read. Whichever you use, rinse and calibrate regularly.

Target ranges

• Reef aquariums — 1.024 to 1.026 specific gravity (34–36 ppt). Coral and invertebrates are sensitive to salinity variation, so consistency within this range is important
  
  
• Fish-only marine aquariums — 1.020 to 1.024 specific gravity. Fish generally tolerate this lower range well, and the reduced salinity makes conditions harder for parasites like ich
  
  
• Brackish aquariums — 1.002 to 1.022 specific gravity. The right level within this wide range depends on the species — research the natural habitat of your fish, as some prefer conditions closer to freshwater while others do best closer to full marine salinity

Why salinity changes over time

Because of evaporation, salinity in your tank will rise gradually over time. Salt does not evaporate — only water does. As water evaporates, the same amount of dissolved salt remains in a smaller volume of water, increasing concentration. This is why topping off your tank with fresh water — not saltwater — is the correct way to compensate for evaporation. Only add new saltwater when doing a water change.

Salinity fluctuations stress fish and invertebrates. Top off with fresh water as often as needed to keep levels stable. For tanks with high evaporation rates, an automated top-off system can keep salinity stable without the daily monitoring.

Salt in freshwater aquariums

Most typical freshwater aquariums do not require salt. However, aquarium salt dissolves into electrolytes — the ions fish need for oxygen uptake and the release of carbon dioxide and ammonia through the gills. When needed, adding a small amount can reduce stress, support disease recovery, and help treat certain internal issues like constipation and mild swim bladder disorders.

Always use salt specifically formulated for aquariums, like Fritz A+ Aquarium Salt, never use regular table salt. Food-grade salt typically contains iodine and anti-caking agents that are harmful to fish and beneficial bacteria.

Aquarium salt does not evaporate and is not filtered out, so it should only be added with each water change. Some aquatic plants are sensitive to salt — use with caution in planted tanks.

Why salt quality matters

Not all aquarium salts are created equal. Raw material quality, elemental ratios, and batch consistency all affect your tank’s health more than most hobbyists realize. A well-formulated salt dissolves clearly, hits stable chemistry quickly, and covers the full range of elements your tank needs. Poor quality salt is a common source of problems that are hard to diagnose.

Fritz salt products

• RPM Complete Marine Salt
• A+ Aquarium Salt
• African Rift Lake Cichlid Salt