Mix salt and water in the right ratio to create saltwater for marine tanks

Saltwater for a marine aquarium is made by dissolving aquarium salt in dechlorinated water. The standard ratio is about 1.020 to 1.025 specific gravity, which translates to roughly 1.5 tablespoons of aquarium salt per gallon of water, though the exact amount depends on the salt brand and your target salinity. You measure the result with a hydrometer or refractometer to confirm the salt concentration is correct before adding it to your tank.

The process takes about 24 hours from start to finish because the salt needs time to fully dissolve and the water needs to reach the right temperature and stability. Rushing this step or using the wrong type of salt can harm fish and corals, so the extra time is worth it.

Key Takeaways

  • Use only aquarium salt designed for marine tanks, never table salt or rock salt, because additives in other salts can poison your fish.
  • Dechlorinate your water first by letting tap water sit for 24 hours, using a dechlorination product, or using reverse osmosis water to remove chlorine and chloramine.
  • Mix salt and water in a separate container and let it circulate for at least 12 hours before testing the salinity with a hydrometer or refractometer.
  • Target a specific gravity between 1.020 and 1.025 for fish-only tanks, or 1.023 to 1.025 for reef tanks with corals.

Choose the right salt for your tank type

Not all salt works for aquariums. Aquarium salt is formulated to include the trace elements and minerals that marine fish and corals need. Common brands include when ready Ocean, Red Sea, and Fritz. These come as dry mixes that you dissolve in water.

Table salt, rock salt, and sea salt from a grocery store contain iodine, anti-caking agents, or other additives that will poison your fish. Even "natural" sea salt sold for cooking is not safe because it lacks the balanced mineral profile of aquarium salt and may contain contaminants.

If you keep only fish and no corals, a basic aquarium salt works fine. If you plan to keep corals or anemones, choose a salt mix labeled for reef tanks, such as Red Sea Coral Pro or Tropic Marin. These contain higher levels of calcium and other elements corals need to build their skeletons.

Prepare your water before adding salt

Tap water contains chlorine and chloramine, which are toxic to fish. You must remove these before mixing in salt. The simplest method is to let tap water sit in an open container for 24 hours — chlorine will evaporate on its own, though chloramine takes longer and may not fully leave this way.

A faster option is to use a dechlorination product such as Seachem Prime or Kent Marine. These neutralize chlorine and chloramine when ready. Follow the bottle instructions for the amount to add per gallon.

The best water for saltwater tanks is reverse osmosis (RO) water, which has had nearly all dissolved solids removed. You can buy RO water at most aquarium stores or grocery stores, or make it at home with an RO system. RO water gives you the most control over salinity because it starts with almost nothing in it.

Mix salt and water in the correct ratio

Use a separate container — a bucket, large pitcher, or spare tank — to mix your saltwater. Never mix salt directly in your display tank. Fill the container with dechlorinated water at the temperature you want your tank to be, usually between 75 and 78 degrees Fahrenheit.

Add aquarium salt according to the package instructions. Most brands recommend 1.5 to 2 tablespoons per gallon, but this varies, so read the label. Stir well to begin dissolving the salt. The water will look cloudy at first — this is normal.

If you have a powerhead or circulation pump, place it in the bucket and run it for at least 12 hours. If not, stir the water every few hours. The salt needs time to fully dissolve and distribute evenly. After 12 to 24 hours, the water should be clear and the salt completely dissolved.

Test salinity with a hydrometer or refractometer

A hydrometer is a floating tube that measures specific gravity — the density of saltwater compared to fresh water. Hydrometers are cheap (under $10) but less accurate than other tools. A refractometer is more precise and works by measuring how light bends through the water. Good refractometers cost $30 to $50 but last for years.

For fish-only tanks, aim for a specific gravity of 1.020 to 1.023. For reef tanks with corals, target 1.023 to 1.025. If your salinity is too high, add more dechlorinated fresh water and retest. If it is too low, add more salt, stir, wait a few hours, and test again.

Rinse your hydrometer or refractometer with fresh water after each use and let it air dry. If you use a refractometer, calibrate it with distilled water once a month to keep readings accurate.

Add the saltwater to your tank gradually

Once your saltwater is mixed and tested, you can add it to your display tank. If you are filling a new tank for the first time, pour the saltwater in slowly over 15 to 20 minutes to avoid disturbing the substrate and decorations. If you are doing a water change on an established tank, add the new saltwater at the same rate you would add fresh water — slowly, so fish and corals do not experience a sudden shock.

Test the salinity of your display tank after the water has mixed for a few minutes. The reading should match what you measured in your mixing container. If it does not, wait another 10 minutes and test again, because the water in the tank may not be fully mixed yet.

Maintain consistent salinity between water changes

Saltwater evaporates, but salt does not. As water leaves your tank, salinity rises. You must replace evaporated water with fresh dechlorinated water, not saltwater. Check your salinity once a week with a hydrometer or refractometer. If it has crept above 1.025, add fresh water until it drops back to your target range.

During a water change, remove the old saltwater and replace it with newly mixed saltwater at the same salinity as your tank. Most aquarists do a 20 to 30 percent water change every two to four weeks, depending on how many fish and corals they keep and how much they feed.

Keep a log of your salinity readings and water changes. This helps you spot trends — for example, if salinity is rising faster than evaporation alone would explain, it may mean your salt mix is too concentrated, or you need to adjust your measuring method.

Frequently Asked Questions

Can I use table salt or sea salt from the grocery store?

No. Table salt contains iodine and anti-caking agents that poison fish. Grocery store sea salt lacks the trace minerals marine life needs and may contain contaminants. Always use aquarium salt formulated for marine tanks.

What if my saltwater is too salty or not salty enough?

If too salty, add dechlorinated fresh water and retest after mixing for a few hours. If not salty enough, add more aquarium salt, stir, wait, and retest. Make small adjustments — add a tablespoon or two at a time — so you do not overshoot your target.

How long does mixed saltwater stay good before I use it?

Saltwater mixed in a bucket can sit for weeks if you keep it covered and do not let algae grow in it. If you mixed it with a powerhead running, turn the powerhead off once the salt is fully dissolved to avoid unnecessary aeration. Store it in a cool, dark place.

Do I need special equipment to make saltwater?

At minimum, you need a bucket, aquarium salt, dechlorinated water, and a hydrometer or refractometer to test salinity. A powerhead or pump speeds up mixing but is not required — you can stir by hand every few hours.

Why does my refractometer give a different reading than my hydrometer?

Refractometers are more accurate than hydrometers. If readings differ, trust the refractometer. Hydrometers can drift over time or give false readings if they are not perfectly vertical in the water. If you rely on a hydrometer, replace it every year or two.