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Salt Mix Guide

Clean, pure saltwater is crucial to the health and well-being of creatures in your marine aquarium.  Seawater is composed of many different ions (salts) in different concentrations. The main ions are sodium (Na), chloride (Cl), magnesium (Mg), and sulfate (S04), forming the salts (NaCl and MgS04).  Salinity is the measure of the concentration of dissolved salts (ions) in the water, often given in ppt – parts per thousand.  Normal seawater is about 35 ppt salts.

But sea water is not just “salty”; it contains a precise mix of trace elements, all of which should be present in an artificial replacement. Using a commercial salt mix, you create water for the aquarium that is in many ways superior to natural sea water, with buffers to help maintain the proper pH, as well as additional calcium and other trace elements to maintain corals, clams and other reef invertebrates.

In a closed system like an aquarium, the supply of certain dissolved substances can become severely depleted between water changes with no natural replenishment as would occur in the ocean. In a healthy reef tank with rapidly growing stony corals and tridacnid clams, the demand for calcium, magnesium and other trace elements can be so high that salt mix alone will not provide enough. Such a system can require almost constant supplementation.

Measuring Salinity

It is important to know the salinity of your tank and any new seawater you make before topping off. There are a number of tools used to measure salinity, but a simple hydrometer or refractometer will give you accurate results. These devices provide values for specific gravity or salinity. Ideally you’ll be looking for a salinity of 34 to 36 ppt or a specific gravity from 1.021 to 1.026.

Safe Water to Use for Mixing Saltwater

Making saltwater for your aquarium begins with finding a pure water supply. Tap water, whether drawn from a well or from a municipal supply, can contain various chemicals or dissolved minerals that can be harmful to your tank. There are many water filtration systems available that will produce purified water for use in your aquarium, which may be the safest solution. Many aquarists prefer RODI units that combine reverse osmosis (RO) and deionization (DI) to remove impurities and undesirable compounds from tap water.


Choosing Salt Mixes

Next, choose a salt mix. Not all salt is created equal in the aquarium world. General aquarium salt, which is simply sodium chloride and other specialized mixes such as cichlid salt are not suitable for a marine or reef set-up as they do not include the ions necessary for oceanic organisms to thrive. Even each brand of marine salt mix varies from natural seawater’s concentrations of the four major ions (sodium, chloride, sulfate and magnesium), as well as other trace elements. The following chart is a comparison guide for Calcium, Alkalinity, and Magnesium  levels in our most popular artificial marine aquarium salt mixes.  All readings are based upon  mixing salt water to a concentration of 35 ppt or a specific gravity of 1.026.  Concentrations are based upon published manufacturers product literature.


BrandCalcium (ppm)Alkalinity (dkh)Magnesium (ppm)
Instant Ocean400111350
Reef Crystals490131440
Kent Marine5408.51200
Red Sea4107.71280
Red Sea Coral Pro45012.21340
Seachem Reef540101450
Tropic Marin37510 1230
Topic Marin Pro Reef4508.51380
Fluval Sea4908.61425


Mixing Saltwater

Follow the manufacturer’s directions on how much mix to use, and check the solution for proper salinity as you add mix. Add more salt mix to raise the salinity or more fresh water to lower salinity. Make sure all salt is completely dissolved before testing to ensure accurate readings.  Altering salinity of your tank significantly may also alter alkalinity, calcium, magnesium, and other ions to the point where they will need adjustments, so be sure to have test kits on hand to monitor these levels.

Never mix the salt in the aquarium.  Start with a large vessel (large buckets or plastic trash cans work well) of reverse osmosis (RO) or distilled water. Slowly add salt to the water, rather than putting the salt in first and then filling the bucket to prevent precipitation and clumping.  Use an air stone or power head in the water to circulate and aerate the mixture. Use a refractometer or hydrometer to measure the salinity and adjust accordingly, you want to match the salinity of your aquarium if you’re performing a water change. You may also add calcium, magnesium, alkalinity buffers or other elements after the salt has dissolved.

Maintaining Your Saltwater Tank

Water changes should be performed regularly on your marine aquarium. 20-25 percent of the water should be replaced per month, ideally about 10 percent every other week or as needed according to water chemistry test results. Mix your new salt water a day or two in advance. Once mixed, artificial seawater can be stored for as long as needed without continuous stirring, and having a small reservoir of mixed saltwater ready to use provides extra insurance in case of emergency or unforeseen accidents. Replenish evaporated water with clean fresh water, as salts remain when water evaporates. And remember, replenishing or topping off the tank with fresh water does not qualify as a water change, as accumulated waste materials are not removed.