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How to Make Saltwater

The water quality and parameters of a saltwater aquarium directly impact the health of the resident organisms. Natural ocean water contains dissolved gasses, salts, minerals, trace elements and other substances in specific ranges. Artificial saltwater should aim to mimic the parameters of natural ocean water. The contents of ocean water are listed at the end of this article, and the major parameters of concern to aquarists can be found here.  

Selecting the Source Water

Selection of appropriate source water is a foundational step toward establishing the healthiest possible environment in your aquarium. RO/DI (Reverse osmosis and deionized) and distilled water are the purest source water products available. These types of water contain very few to no total dissolved solids (TDS). These types of source water set a blank slate that allows you to have control over the salt, minerals and other trace elements going into your tank. RO/DI source water contains few excess nutrients that can support unwanted microbial activity in your tank such as algal or bacterial blooms. Reverse osmosis systems (without a deionization step) are also pure enough to use in a reef aquarium but may contain dissolved salts and trace amounts of fluoride, calcium, chlorine, magnesium and sulfate.

RO/DI water may be available at your local fish store, and distilled water is available at nearly all grocery stores. If your tank is large, you may prefer to purchase your own filtration system to produce water at home. Carrying multiple gallons of water in containers can become tiring very quickly.

Tap water, bottled spring water, and other commercial drinking water should be avoided if possible. If used, these should be filtered to remove chlorine, chloramine, and excess minerals and nutrients. Tap water may contain chloramine, which is more difficult to remove than chlorine. Both are toxic to most (desired) aquarium organisms. In addition, drinking water should contain dissolved solids, but the source water for your aquarium should not. Human hydration requires more than H2O; our bodies require salt and other electrolytes to maintain our hydration. Marine salt mixes contain salt, minerals and trace elements in proportions similar to natural seawater. If you add marine salt mix to water that already contains minerals and electrolytes, the mixed saltwater will contain these in excess.

Well water contains dissolved gasses that can be very harmful to aquatic organisms. Hydrogen sulfide is a toxic gas that binds to hemoglobin and prevents the transmission of oxygen throughout an animal’s circulatory system. Dissolved carbon dioxide also decreases pH.

If you plan to use these water sources, you should have them lab tested to see what you are adding to your aquarium. Semi-annual testing may be needed because dissolved solid content of water from these sources fluctuates over time.

Selecting Salt

Use only marine salt mix to make your saltwater. Rock salt usually does not contain the minerals or trace elements required for many saltwater organisms such as invertebrates and coral. Table salt is not appropriate for aquarium use.

Marine salt mixes formulations vary by brand, and different varieties and brands of salt may offer different advantages for the maintenance of the organisms in your aquarium. 

For example, stony corals consume high quantities of calcium. Salt mixes formulated for reef aquaria often better support these needs. Regular water changes of saltwater containing reef-formulated aquarium salt may meet your stony corals’ calcium consumption needs without frequent calcium supplementation. All major brands are safe for fish and shouldn’t cause harmful fluctuation in your system.

Storing and Measuring Salt

Packaged salt should be stored in a dry and dark environment.

Upon opening, the entire salt package purchased should be mixed to the manufacturer's provided recipe and stored in a clean, sealed water container. Prior to use, the mixed saltwater should be stored for a few days. Water changes can be stressful for delicate aquarium inhabitants, and chemical reactions and unmixed salt cause further disturbance during a water change. (If you stir the water with your hand - which is less than ideal - you may notice heat produced by these reactions when the salt is mixed.)

You can add graduated markings on your saltwater storage containers to help keep track of how much saltwater you have available for water changes. You will have to agitate the salt to get it to mix fully, and a 5 gallon bucket is not sufficient to make 5 gallons of saltwater. 

Always store unused salt in sealed bags or containers. Salt in unsealed storage can attract environmental pollutants and will accumulate any available moisture, especially in environments with high humidity. The added weight and volume of the moisture increases the indicated weight and volume of the salt, which can lead to inaccurate mixing later. Measure the salt placed in each bag or container directly after opening the bag to facilitate accurate mixing later.

Accurate measurements occur when salt is dry, before it has accumulated moisture. Because of the compaction of some salt crystals, weight measurements are more precise than volume measurements. Use a postage scale that is accurate under 25 pounds. Don’t forget to use the tare feature. Use containers that are inert to salt, and clean the containers before use.

Mixing Salt

Add a known amount of salt to a measured amount of water. Slow, gradual salt addition is more efficient than dumping in all the salt at once. Finish mixing the salt with manual agitation, a powerhead, aggressive aeration, and/or a pump that feeds back into the container. All the salt should be dissolved before the water comes into contact with aquatic organisms. If mixing the salt manually, use a clean plastic implement to stir. Minimize contact of the water with your hands. Saltwater can remove dirt and oil from hands. These contaminants go directly into the saltwater.

Testing the mixed Saltwater for Suitability

It is good practice to test all water going into your aquarium before use. Generally, seawater has a specific gravity of 1.025 and a salinity of 35 parts per thousand. The target salinity of new saltwater is determined by the needs of aquarium inhabitants and the aquarist’s maintenance routine. Common salinity measures include hydrometers, refractometers and electrical conductivity meters. These tools are listed in the same order as their ascending price. 


A hydrometer indicates the specific gravity of a liquid. The arm in the hydrometer rises as the density of the liquid increases. Fill the hydrometer to the marked line with the mixed saltwater to read the specific gravity of your sample. Make sure no trapped air bubbles are raising the arm as these will result in an erroneously high reading. Rinse your hydrometer thoroughly between uses. Any residue or saltwater left in the hydrometer after use has the potential to affect your next reading.


A refractometer may be the most popular tool for hobbyists measuring aquarium salinity. A refractometer uses light to measure the refractive index of a sample. Place a small amount of saltwater on the plate, close the flap, and point the end of the refractometer toward a light source while looking through the eyepiece. Most refractometers display the result in specific gravity and the salt concentration in parts per thousand.

Refractometers should be regularly calibrated to ensure accuracy. After you become accustomed to a regular salt mixing routine, you may limit calibration to times when the mix does not match your expected results. Use a calibration solution (saltwater of a known salinity) as you would use a sample of your mixed water. If the known salinity of the calibration solution is the same as the salinity indicated by the refractometer, then your refractometer is properly calibrated. In the event of a different reading, adjust the calibration knob with a screwdriver until the salinity reading is correct.

Conductivity Meter

These instruments measure the electric conductivity of water to indicate salinity. They produce fast and accurate readings and can be used to constantly monitor salinity. Continuous conductivity monitors are best suited as alert (rather than control) devices. The alert allows the aquarist to assess the situation and take appropriate action.

Parameter Matching 

Rapid changes in temperature and pH cause stress for many aquatic organisms. Even if the temperature and pH of your new saltwater parameters are within healthy ranges for your aquatic life, a difference between the parameters of the new water and the parameters within the tank can harm your aquarium inhabitants.

The temperature of newly mixed water may be adjusted with a heater or ambient conduction. pH may be adjusted with acidic or alkaline buffers. Kalkwasser (dissolved calcium hydroxide) can be used to raise pH. It also contains calcium, so be sure to check your calcium level after using kalkwasser. Other products can be used to add alkalinity (carbonate hardness) and magnesium. Always follow the manufacturer’s directions for the product.

Adding Water to the Tank  

Dumping water into a tank may disturb the sandbed and shock or physically buffet tank inhabitants. Slow addition by siphon is a safer method, especially for reef tanks and delicate invertebrate animals. One way to accomplish this is to place new water in a container above your sump and siphon the water from the container into the tank. ¼” airline tubing is usually appropriate for a slow siphon. You may choose to adjust the siphon speed if the siphon is disturbing any reef organisms or settled particulates. The sump is usually the best place to add new saltwater because it will mix with the system water before it is sent into the main display.

Maintaining Consistent Salinity

Water loss through evaporation is an inevitable process in home aquaria. When water evaporates, the salt remains in the tank, resulting in a gradual increase in salinity. To maintain a healthy salinity, you will have to replace the freshwater that evaporates. Many aquarists use an automatic top off system to add RO/DI water to the aquarium as evaporation occurs. These systems use a float switch to detect when the water level in your aquarium has dropped. When the float falls, a pump within the RO/DI reservoir fills the aquarium system until the float rises again. These systems depend on the float valve to prevent overfilling the tank and causing an overflow. If you have snails or other animals that could reach the float, be sure to block their access to it.

Manual RO/DI addition eliminates the risk of failed float valves, but the range of salinity levels in the aquarium will be more variable.


  • Start with the purest source water possible
  • Use marine salt mixes to make saltwater, follow manufacturers directions
  • Check salinity, temperature, and pH before adding new water to your aquarium
  • Add water to the tank slowly, so as not to stir up detritus or disturb the inhabitants

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