Mixing Saltwater

Whether you are filling up a brand new aquarium or are making up a batch of saltwater for a water change, the procedure for mixing saltwater is exactly the same.

Many beginning aquarists use dechlorinated tapwater.  Unfortunately, while the dechlorinator takes care of the chlorine in tapwater, it does not remove nitrates, phosphates, metals and other impurities.  Nitrates and phosphates will simply fuel algae blooms.  Get a TDS (Total Dissolved Solids) meter, or ask your LFS if you can bring in your tapwater for testing.  Household tapwater has TDS ranging from the 60s to 100s.

Always use Reverse Osmosis (RO) or De-Ionized (DI) water for mixing saltwater!

If setting up a large new aquarium, it also possible to arrange for several dispenser bottles from a company that sells distilled water for drinking.  But make sure that it is distilled water and not simply ‘purified’ water.  RO/DI and distilled water should have a TDS reading of zero, or as near zero as possible.

Add your RO/DI or distilled water to your mixing container first.   Aquarium salt should be added to water and not the other way around!   If water is added after salt has been put in the container it will cause calcium in the salt to precipitate and the water will turn milky white, making it impossible to use.

At this stage, it is a good idea to stir the mixture to make the salt dissolve as much as possible.  The long plastic handle of an algae scraper comes in handy here!  Watch out for that sharp blade though.

After a good stirring, put in a powerhead which will help mix the new saltwater thoroughly and aerate it as it is mixing.  And have a dedicated powerhead for this job as well.

Mixing Saltwater And Specific Gravity

When mixing saltwater, add enough salt to the water so that a specific gravity (SG) of 1.021 to 1.026 is reached.  Check the specific gravity with a refractometer for accuracy and make a note of the amount of salt you need to bring up the water in your container up to the salinity you need.

Don’t worry if the salt hasn’t dissolved completely yet — it is already within range.  I have a designated cup for measuring out the amount of aquarium salt I need — in my case, it is exactly 8 cups for an SG of 1.025.  Note that ‘cups’ here does not denote an actual unit of measurement.

Fish Only With Live Rock (FOWLR) aquariums can be kept at a lower salinity.  In fact, it is preferred when keeping a large number of fish as there is more dissolved oxygen at a lower specific gravity.

If keeping a reef tank with corals, salinity should be between 1.024 to 1.026.  Some corals, especially SPS, do not do well at salinities below 1.024.

Let the water mix for at least 24 hours before using it.  New saltwater with tiny grains of undissolved salt is caustic and will irritate corals and the gills of fish.  Grab a pinch of salt and put your fingers under a slow running tap and you will feel the salt actually heat up and ‘burn’.

It is normal for the new saltwater to be cloudy for a few hours after it is made but it should clear up completely in 24 hours.

A Note About Storage Containers For Mixing Saltwater

Buy a clean plastic container — food grade plastic is best — that can hold at least 10% of the total water volume of your aquarium.   Choose a container with built-in wheels so that you won’t have to lug a heavy container of saltwater from your mixing area to your tank.

Dedicate this container to only to mixing saltwater and nothing else.

And never, ever, use a container or bucket that has once been used with cleaners, soaps or detergents of any kind.

How Much Salt Mix To Buy?

If you’re mixing saltwater for a brand new aquarium, you will need about 13 pounds of salt per 50 gallons.

Conveniently, salt mixes are packaged in boxes, buckets or bags in various sizes.  The manufacturer will indicate on the packaging how much saltwater, in gallons, it will yield.  Tropic-Marin brand salt mix for example, has packages for 10, 32, 52, 150, 200 and 300 gallons.

To calculate the gallonage of your aquarium multiply Length x Width x Height in inches and divide the total by 231.

If you are buying saltwater for routine water changes, buy as much as you would like to store for future use.  Buying in larger quantities will save you a few more dollars.  It is also important to have additional salt on hand should an aquarium emergency arise.

Storing Aquarium Salt

Aquarium salt should be stored in a cool, dry place and away from direct sunlight.  It will be easier to store aquarium salt if it comes in a resealable plastic bucket as opposed to a cardboard carton.  And that plastic bucket will come in useful for chores like water changes, once you’ve used up the salt.

Some aquarists store their aquarium salt together with a small mesh bag filled with a small handful of uncooked rice.    The rice acts as a natural drying agent and prevents the salt from clumping or hardening into a solid brick.  Avoid chemical drying agents as they might get into the salt.

More pages on Saltwater Aquarium Maintenance:

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