salinity & saltwater

Reef Tank Chemistry – Salinity

There are a number of parameters you need to monitor to maintain a healthy and successful reef tank. Among the most critical are Alkalinity and pH, Salinity, Temperature, Calcium, Magnesium, Phosphate, and Ammonia. There is a second tier of parameters less critical but still important to know about and may require monitoring depending on your specific tank type. This discussion covers Salinity.

Salinity – is the measure of the concentration of dissolved salts (ions) in water. Normal seawater is about 3.5% salt and 96.5% water by weight. Salinity is often given in ppt – parts per thousand. A salinity of 35ppt is just another way to say 3.5% salt. (35 ÷ 1000 = .035 then multiply by 100 to convert to percent = 3.5%)

Seawater is composed of many different ions (salts) in different concentrations but the sum of them all adds up to 3.5%. (For a detailed breakdown see The main ions in seawater are sodium (Na), chloride (Cl), magnesium (Mg), and sulfate (S04), forming the salts (NaCl and MgS04). It is these that determine the salinity by how much is present. And even thought there are many other ions in seawater, changes in their concentrations have little impact on salinity, but they may be important in other ways.

Measuring Salinity

You need to measure the salinity of your tank and any new seawater you make before adding it to your tank. There are a number of methods used to measure salinity: Refractometers, hydrometers, and conductivity probes. These devices typically report values for specific gravity (which is unit less) or salinity (ppt ) and less commonly used conductivity (in milliSiemens (mS) per centimeter (cm)). Depending on the instrument you use, your target values will be:

  • Salinity = 35 ppt,
  • Specific gravity = 1.0264
  • Conductivity = 53 mS/cm

Don’t worry too much about small deviations; salinity is fairly forgiving in most reef tanks. A salinity of 34 to 36 ppt or a specific gravity from 1.021 to 1.026 is a good target. If the ppt or specific gravity is too high, you can lower it by removing some of the tank water and replacing it with just RO/DI water. If too low you can add more salt mix. You will see recommendations to keep the salinity somewhat lower if you have fish – 1.019 to 1.023 as they do better at lower salinity. Do know that altering salinity significantly does alter the alkalinity, amounts of calcium, magnesium, and other ions to the point where they will need adjustments.

Making Artificial Seawater

Clean, pure saltwater is crucial to everything in your marine aquarium. Many problems are avoided by starting out with the correct type of water you use to make your salt water and by and properly mixing saltwater. The water from your tap contains much more than water and what it contains varies with its source. Well water contains naturally dissolved minerals, including hydrogen sulfide, bad for your tank and its inhabitants. Water from municipal sources contains chemicals, like chlorine and chloramines that make it safe for human consumption but are bad for your tank and even in low concentrations, will burn the gills of your fish. The EPA standard for Nitrate (as NO3-N) allowed in municipal water is 10.0 mg/l, over twice the recommended maximum level for aquaria. Additionally heavy metals and copper, allowed in tap water at levels as high as 1.3 mg/, can harm invertebrates in reef tanks. Even your house pipes can add contaminants detrimental to your tank.

For most aquarists, seawater is something you will have to make by mixing a
commercial salt product designed for salt water aquarium with suitably purified
freshwater. “Sea salt” sold to consumers for purposes like cooking cannot be
used; it does not have the right mix ions that match sea water!

Which Salt Mix to Use?

First realize that every artificial salt mix varies from natural seawater’s concentrations of some ions, especially when it comes to minor and trace elements. The four big ions (sodium, chloride, sulfate and magnesium) must be added in large concentration to create seawater. Aquarium salt manufacturers need to balance purity with cost; the purer the ions, the more expensive the mix will be. It is inevitable that there are bound to be some impurities. Salt mixes have been analyzed1 but when you look through the information no one brand really matches the make up of seawater. A good solution is one proposed by Randy Holmes-Farly who surveyed which salt ‘Tank of the Month” winners used and suggests “picking one of the top contenders because it can produce a beautiful and successful aquarium.”2.

Start With the Right Water

The safest solution is to put the water through a purification process before it is added into any tank. The preferred method by most aquarists is a series filter system that combines reverse osmosis (RO) and deionization (DI). Reverse osmosis is the process of forcing a solvent (here, water) from a region of high solute (the impurities) concentration through a semipermeable membrane to a region of low solute concentration by applying pressure. The semipermeable membrane keeps the ions and compounds we don’t want on one side while allowing “pure” water to pass through. Deionization removes charged compounds by filtering the water through a deionizing resin. There are also sediment filters and activated carbon filters that are often used in conjunction with a RO/DI system to keep these parts operating properly. A typical water filtration system would filter water in this order:

  • Sediment Filter: This does exactly what its name implies: filter sediment. Sediment can clog activated carbon filters and damage RO membranes.
  • Activated Carbon: Primarily used to break down chlorine and chloramine and possibly other organic chemicals.
  • Reverse Osmosis (RO): Purifies based on the size of molecules allowed through the membrane, the goal being allow water molecules through and anything larger stays behind. It does need to be noted that a number of compounds of concern to reef aquarist can pass through a reverse osmosis membrane to some extent: carbon dioxide (CO2), ammonia (NH3), hydrogen sulfide (H2S) and silicic acid (Si(OH)4). All of these should be trapped by the DI resin.
  • Deionization Resins (DI): These trap all charged molecules passing through, allowing just neutral (uncharged) molecules to pass through. The resin consists of porous beads that have fixed charges attached to them and different beads are used to bind cations (positive charge) and anions (negative charge).

There are many water filtration systems available for purchase that will produce purified water for use in your marine, reef, or fresh water tank. You may also be able to purchase RO/DI water from a local pet or aquarium retailer or similar source.

Mixing your salt water

Unless your tank residents are from an environment you know to be substantially different from that of natural seawater, like brackish with a lower salinity or from the Red Sea that has a higher salinity, mixing to 35ppt salinity will be adequate for most tanks.

  • You will need a mixing container that holds at least 5 to 10 percent of the volume of water your display tank holds – i.e. if you have a 100 gallon tank, your container needs have the ability to hold at least 5 – 10 gallons. Containers can be of any size but the larger it is, the more constant your mix will be in its parameters. Old aquariums, clean storage containers, and new garbage cans are popular choices.
  • Fill the container with your RO/DI freshwater.
  • It’s a good practice to aerate RO water for 12-24 hours before mixing in the salt. This will help drive off excess CO2 so you get a proper pH and buffering capacity in your mixed seawater.
  • Add the salt following the manufacturer’s instructions. Your freshly mixed seawater is very caustic and needs time to cure, a minimum of 24 hours, before it can be used in your aquarium. While it cures, use an airstone or powerhead in the water to circulate and aerate the mixture and put in a heater to make it match the water temperature of your display tank.
  • You want to match the mixing tank salinity to that of your aquarium. Use a refractometer to measure the salinity and adjust accordingly. (Ideally, salinity should be 27 to 35 parts per thousand (ppt), or 1.020 to 1.026 specific gravity)
  • If you want to add any calcium, magnesium, alkalinity or anything else, add it after the salt has dissolved.
  • You spend a lot of time and money on your tank. Having extra saltwater ready to use provides extra insurance for those unforeseen accidents and the minimal investment in a container, powerhead, and heater is well worth the small cost and effort.
  • Mixed artificial seawater can be stored for as long as needed without continuous stirring or heating when made from adequately pure freshwater.

Reef Tank Salinity Maintenance Tips

  • Before changing your aquarium water, measure and record the specific gravity.
  • Make sure the new saltwater matches the specific gravity of your aquarium. Too much of a deviation in salt levels could “shock” or potentially kill sensitive aquarium inhabitants.
  • Replenish evaporated water with clean freshwater – not prepared salt water. Remember, salts remain behind after water evaporates. If you top off your aquarium with salt water, you are compounding salt levels in your aquarium!


1,2 “Reef Aquarium Chemistry for Beginners, Part 1: The Salt Water Itself” by Randy Holmes-Farley (

How to make a salt water mixing container:

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