Mix salt and water in the right ratio to create seawater for your tank

Seawater for an aquarium is made by dissolving marine salt mix into freshwater. The salt mix contains not just sodium chloride, but also the minerals and trace elements that saltwater fish and corals need to survive. You cannot use table salt or sea salt from the grocery store—those lack the right balance of calcium, magnesium, and other compounds. A proper marine salt mix comes in a bag or bucket and costs between $20 and $60 depending on the brand and size.

The standard ratio is one part salt mix to roughly 35 parts water by weight, which creates water with a salinity of about 1.025 specific gravity. This is the measurement aquarists use instead of percentage. You will need a hydrometer or refractometer to check your work—a hydrometer costs $10 to $20 and floats in the water to show you the salinity reading. A refractometer is more accurate but costs $30 to $100.

Key Takeaways

  • Marine salt mix is not the same as table salt or sea salt; it contains the minerals fish and corals need, and you must buy it from an aquarium supplier.
  • Mix salt into freshwater (not tap water, which contains chlorine) at a ratio of about 1 part salt to 35 parts water by weight to reach 1.025 specific gravity.
  • You need a hydrometer or refractometer to measure salinity and confirm your mix is correct before adding it to your tank.
  • Let the mixed saltwater sit and circulate for at least 24 hours before using it, so the salt fully dissolves and the water stabilizes.

Choose the right water to start with

Start with freshwater, not tap water from your sink. Tap water contains chlorine and chloramine, which harm fish and corals. You have three options: reverse osmosis (RO) water, distilled water, or deionized (DI) water. All three remove the chemicals and minerals that make tap water unsuitable.

Reverse osmosis water is the most common choice for saltwater aquariums. You can buy it at most aquarium shops for $0.50 to $1.50 per gallon, or rent or buy an RO unit for your home if you plan to maintain a tank long-term. Distilled water and deionized water work just as well but are harder to find in large quantities and cost more per gallon. If you have a water softener at home, its output is not the same as RO or DI water and should not be used.

Measure and mix the salt correctly

Weigh both the salt and the water using a kitchen scale. This is more accurate than measuring by volume. For every 35 ounces of freshwater, add 1 ounce of salt mix by weight. If you are making 10 gallons (about 80 pounds or 1,280 ounces), you would add roughly 36 ounces of salt mix.

Pour the freshwater into a clean bucket or mixing container first. Add the salt mix slowly while stirring. The salt will take time to dissolve fully—you may see it clumping or clouding the water at first. Keep stirring until all visible crystals are gone and the water looks clear. This usually takes 5 to 10 minutes of steady stirring, though some salt mixes dissolve faster than others.

Do not use a metal bucket or container; use plastic or glass. Metal can react with the saltwater and introduce unwanted elements into your mix. Make sure the bucket is clean and has never held soap, chemicals, or anything other than water or aquarium supplies.

Check the salinity with a hydrometer or refractometer

Once the salt is fully dissolved, measure the specific gravity of your mix. A hydrometer is a glass tube weighted at the bottom that floats in water. The higher it floats, the saltier the water. You want a reading of 1.023 to 1.026 specific gravity; 1.025 is the standard target. Float the hydrometer in your bucket and read the number where the water line crosses the scale.

A refractometer is a small handheld device that measures salinity by looking at how light bends through a drop of water. You place a few drops on the lens, look through the eyepiece, and read the salinity directly. Refractometers are more accurate than hydrometers, especially in smaller tanks, but they cost more and require calibration with freshwater before each use.

If your salinity is too low, add more salt mix and stir again. If it is too high, add a small amount of freshwater and retest. Make small adjustments—add salt or water in small amounts and retest rather than guessing at large corrections. It is easier to get it right the second time than to overcorrect.

Let the water circulate and stabilize for 24 hours

Before you pour the saltwater into your tank, let it sit in the mixing bucket with an air stone or powerhead running. An air stone is a small porous stone that bubbles air through the water; a powerhead is a small pump that creates circulation. Either one helps the salt fully dissolve and allows gases to equilibrate. Leave it running for at least 24 hours.

During this time, check the salinity again. It may shift slightly as the water temperature changes or as any remaining salt crystals dissolve. If the reading has drifted outside your target range, adjust it now before adding the water to your tank. Once the water is in the tank with fish or corals, adjusting salinity becomes much harder.

Use the saltwater in your tank or for water changes

After 24 hours of circulation, your saltwater is ready to use. If you are setting up a new tank, pour it in slowly to avoid stirring up the substrate or damaging any decorations. If you are doing a water change on an established tank, match the temperature of the new saltwater to the tank water before adding it. A sudden temperature drop can stress fish and corals.

For ongoing water changes, most aquarists replace 20 to 25 percent of the tank water every two to four weeks. You can make saltwater in advance and store it in a sealed container for up to a few weeks, as long as you keep it in a cool, dark place. Before using stored saltwater, check the salinity again—it should not have changed, but it is worth confirming.

Common mistakes to avoid

Do not use tap water, even if you have a water filter at home. Chlorine and chloramine will kill fish and corals, and a standard pitcher filter does not remove them. Do not use table salt, sea salt, or any salt product not labeled as marine salt mix. These lack the trace minerals and may contain additives that harm aquatic life.

Do not skip the 24-hour circulation step. Saltwater that has not fully stabilized can have pockets of different salinity, and adding it to your tank can create stress zones where fish avoid going. Do not assume your salinity is correct without measuring it. Even small errors compound over time as water evaporates from your tank.

Do not mix saltwater in a metal container or with metal tools. Stainless steel is safer than other metals, but plastic and glass are the safest choices. Do not add the salt mix directly to your tank—always mix it in a separate bucket first so you can measure and adjust the salinity before it reaches your fish.

Frequently Asked Questions

Can I use tap water if I let it sit for a few days?

No. Sitting does not remove chlorine or chloramine from tap water. You need reverse osmosis, distilled, or deionized water. If you cannot find these locally, ask your aquarium shop about delivery options or consider renting an RO unit if you plan to maintain the tank long-term.

What if my salinity reading is off by 0.002 or 0.003?

Small variations within 1.023 to 1.026 are normal and acceptable. Fish and corals can handle this range. If your reading is outside this range, adjust it before adding the water to your tank. Once fish are in the water, large swings in salinity stress them, so prevention is easier than correction.

How long can I store mixed saltwater before using it?

Saltwater stored in a sealed, dark container at room temperature stays usable for several weeks. Before using stored water, check the salinity again—it should not have changed, but temperature shifts or evaporation can affect the reading. If you store it longer than a month, test it before adding it to your tank.

Do I need a powerhead or air stone during the 24-hour wait?

Yes. Circulation helps salt dissolve completely and allows the water to reach chemical equilibrium. Without it, you risk pockets of different salinity in your bucket. A straightforward air stone and air pump (around $15 to $30) is enough for most home mixing setups.

What brand of salt mix should I use?

Common brands include when ready Ocean, Red Sea, and Seachem. All are formulated for home aquariums and produce water in the correct salinity range when mixed at the standard ratio. Your aquarium shop can recommend a brand that works well with your local water and tank type. Cost and availability vary by location.