Mix salt and water in the right ratio, then test before adding fish

Saltwater for an aquarium is made by dissolving aquarium salt in fresh water at a specific gravity that matches what your fish and corals need. The most common target is a specific gravity of 1.020 to 1.025, which you measure with a hydrometer or refractometer after mixing. You will need three things: reverse-osmosis (RO) water or distilled water, aquarium salt mix, and a way to measure salinity. Tap water contains minerals and chlorine that harm marine life, so do not use it.

The whole process takes a few hours from start to finish, but most of that is waiting time while the salt dissolves and the water aerates. Once you understand the steps, mixing saltwater becomes routine—most aquarists do it every few weeks when they perform water changes.

Key Takeaways

  • Always use reverse-osmosis or distilled water, never tap water, because tap water contains chlorine and minerals that kill marine organisms.
  • Aquarium salt mix is not the same as table salt or sea salt from a grocery store—buy a brand made for marine aquariums.
  • Specific gravity between 1.020 and 1.025 is the standard range; measure it with a hydrometer or refractometer before adding water to your tank.
  • Let the mixed saltwater sit and aerate for at least a few hours, preferably overnight, before using it in your aquarium.
  • Test the salinity again after the water has been in your tank for a day, because salt settles and salinity can shift slightly.

What water and salt to buy

Reverse-osmosis (RO) water is the best choice. It has had almost all minerals and contaminants removed. You can buy it by the gallon at most aquarium stores, pet stores, or some grocery stores with water stations. Distilled water works as a second choice, though it is slightly less pure. Do not use tap water under any circumstances—chlorine will kill your fish, and minerals will throw off your salinity readings.

Aquarium salt mix is a powder that contains not just salt but also the trace elements and minerals that marine fish and corals need. Common brands include when ready Ocean, Red Sea, and Fritz. These are formulated to dissolve completely and reach the right specific gravity when mixed at the ratio printed on the bag. Table salt, sea salt from a grocery store, and rock salt are not suitable—they lack the additives and may contain iodine or anti-caking agents that harm marine life.

The basic mixing process

Start by filling a clean bucket or container with RO water. The container should be large enough to hold the amount of saltwater you need, plus a little extra for testing. A 5-gallon bucket works well for most home aquariums. Pour the water in first, then add the salt mix according to the ratio on the package—usually about 1/2 cup of salt per gallon of water, though this varies by brand.

Stir the mixture thoroughly for several minutes. The salt will not dissolve when ready; keep stirring until you see no more powder settling at the bottom. Use a clean stick, spoon, or aquarium-safe stirrer. Do not use anything that has been in contact with soap, chemicals, or food. Once the salt appears fully dissolved, let the water sit and aerate. The best way is to place an air stone (a small porous stone attached to an air pump) in the bucket and run it for at least a few hours, ideally overnight. Aeration helps the salt dissolve completely and removes chlorine if any tap water accidentally got in. If you do not have an air pump, stir the water every 30 minutes for 2 to 3 hours.

Measuring salinity with a hydrometer or refractometer

A hydrometer is a weighted glass tube that floats in saltwater. The higher it floats, the saltier the water. You fill a tall, narrow container (often a test cylinder) with your mixed saltwater, place the hydrometer in it, and read the number where the water line crosses the scale. You are looking for a specific gravity reading between 1.020 and 1.025. Most hydrometers have this range marked in green. A hydrometer costs $5 to $15 and is the most common tool for home aquarists.

A refractometer is more accurate and faster. You place a small drop of saltwater on a glass plate, close the cover, and look through an eyepiece to read the salinity. Refractometers cost $25 to $50 but are worth it if you mix saltwater often or keep corals that need precise salinity. They also work better in very small volumes of water. If your reading is too low (not salty enough), add more salt mix, stir, aerate for another hour, and test again. If it is too high (too salty), add a small amount of fresh RO water, stir, aerate, and retest. Make small adjustments—it is easier to add salt or water gradually than to overshoot and have to start over.

Temperature and timing before use

Saltwater should be close to room temperature before you add it to your tank. If you mixed it in a cool room and your aquarium is warm, let the bucket sit near the tank for an hour so the temperatures match. A sudden temperature change stresses fish and can cause them to go into shock.

Do not rush to use the water when ready after mixing. Even after aeration, let it sit for at least a few hours—overnight is better. This allows any remaining particles to settle and gives the salt time to fully integrate. Some aquarists let new saltwater age for 24 hours before use. This extra time is not wasted; it is when the water stabilizes and becomes safe for your fish.

Checking salinity after the water is in the tank

Test the salinity again 24 hours after you have added the water to your aquarium. Salt does not distribute when ready, and readings can shift slightly as it settles. If the reading is off, you can adjust by removing some water and replacing it with either fresh RO water (to lower salinity) or pre-mixed saltwater at the correct strength (to raise it). Small adjustments over a day or two are better than large ones that shock the system.

Once your tank is established, test salinity at least once a week. Water evaporates from the tank, but salt does not—so as water evaporates, salinity creeps up. You replace evaporated water with fresh RO water only, never saltwater, to keep salinity stable. This is called topping off, and it is different from a water change.

Common mistakes to avoid

Using tap water is the most common mistake. Tap water contains chlorine that kills fish and bacteria, and minerals that make salinity readings unreliable. Always buy RO or distilled water. Mixing salt into hot water or adding hot saltwater to a cool tank both cause problems. Hot water can damage some salt mixes, and temperature shock stresses fish. Mix at room temperature and let everything equilibrate before use.

Not aerating the water long enough leaves undissolved salt at the bottom of the bucket. When you pour the water into the tank, you pour in unsalted water first and salty water last, which throws off your salinity. Aerate for at least a few hours. Trusting a single salinity reading is risky. Hydrometers can be inaccurate, and refractometers need calibration. Test twice—once before adding the water to the tank and once 24 hours after. If the readings differ, test a third time to see which is consistent.

Frequently Asked Questions

Can I use bottled drinking water instead of RO water?

No. Bottled drinking water often contains minerals added for taste, and some brands contain chlorine. RO water and distilled water are the only safe choices. RO water is cheaper and more available at aquarium stores.

What if I do not have an air pump to aerate the water?

Stir the water by hand every 30 minutes for 2 to 3 hours. An air pump is faster and more thorough, but manual stirring works if you are patient. The goal is to dissolve all the salt and let any chlorine gas escape.

How often do I need to mix new saltwater?

Only when you do a water change. Most aquarists do a 25 percent water change every two to four weeks. Mix fresh saltwater only for that change; do not store saltwater for long periods, as it can lose some of its trace elements over time.

What is the difference between specific gravity and salinity?

Specific gravity measures the density of saltwater compared to fresh water. Salinity measures the amount of salt dissolved in the water. They are related but not identical. Most home aquarists use specific gravity (1.020 to 1.025) because hydrometers measure it directly.

Can I use sea salt from the ocean instead of aquarium salt mix?

No. Ocean water contains bacteria, parasites, and pollutants that will harm your fish. Aquarium salt mix is sterile and formulated with the right balance of minerals. It is the only safe choice.