Mix salt and water in the right ratio for a stable marine tank

Saltwater for an aquarium is made by dissolving marine salt mix into fresh water at a specific gravity between 1.020 and 1.025 — the measurement that tells you how much salt is dissolved in the water. You measure this with a hydrometer or refractometer, tools that cost $10 to $40. The process takes about 15 minutes of active work, then 24 hours of aeration before the water is ready for your tank.

The salt you use matters. Aquarium salt mixes (like when ready Ocean, Red Sea, or Fritz) contain not just sodium chloride but also calcium, magnesium, and trace elements that marine fish and corals need. Table salt or rock salt will not work — they lack these minerals and often contain anti-caking agents that cloud the water. Buy marine salt mix from an aquarium store or online.

Key Takeaways

  • Marine salt mix (not table salt) dissolved in fresh water creates saltwater; the ratio is measured by specific gravity, which should read between 1.020 and 1.025.
  • A hydrometer or refractometer measures specific gravity and costs $10 to $40; you need one to know when your water is mixed correctly.
  • Mix salt and water in a separate container, aerate it for 24 hours, then test the specific gravity before adding it to your tank.
  • Temperature affects specific gravity readings, so measure your water at 77°F (25°C) or adjust your target number if you measure at a different temperature.

Gather your materials and tools

You will need a clean bucket or container large enough to hold the amount of water you are mixing — a 5-gallon bucket works for most water changes. Use a bucket that has never held chemicals, pesticides, or soap. Rinse it thoroughly with fresh water only.

Get marine salt mix from an aquarium store. A 50-pound bag costs $30 to $60 and lasts months in most home setups. You will also need a hydrometer or refractometer to measure specific gravity. A basic hydrometer is cheaper ($10 to $15) but less accurate than a refractometer ($30 to $40). For a beginner, a hydrometer is sufficient. You will also need a thermometer, a powerhead or air stone to aerate the water, and an airline tube if you use an air stone.

Calculate how much salt to add

The amount of salt depends on how much water you are mixing. Most aquarium salt mixes include dosing instructions on the bag — typically around 1/2 cup per 5 gallons of water, though this varies by brand. Read your specific salt mix's label first.

If the label does not give clear instructions, start with 1/2 cup per 5 gallons and measure the specific gravity. You can always add more salt if the reading is too low. Add salt gradually rather than all at once — it is easier to add more than to remove salt from water that is already too salty.

Mix the water and salt step by step

Fill your bucket with fresh water from the tap. If your tap water is very hard (high in minerals) or treated with chlorine, let it sit for 24 hours first or use a dechlorinator product. Pour the measured salt into the water slowly while stirring with a clean stick or spoon. Do not dump it all in at once — stir as you add so the salt dissolves evenly.

Keep stirring for 2 to 3 minutes after all the salt is in. The water will look cloudy at first; this is normal. Place an air stone connected to an air pump into the bucket, or use a powerhead to circulate the water. Let it aerate for 24 hours. This serves two purposes: it helps the salt dissolve completely and it removes chlorine if any is present.

Check the specific gravity and adjust

After 24 hours of aeration, turn off the air stone and let the water settle for 5 minutes. Fill the hydrometer's test tube with water from the bucket — do not take water from the very top or very bottom. Place the hydrometer in the tube and read the number where the water line crosses the scale. The reading should fall between 1.020 and 1.025.

If the reading is below 1.020, the water is not salty enough. Add more salt mix, stir well, aerate for another 2 to 4 hours, then test again. If the reading is above 1.025, the water is too salty. Add fresh water a little at a time, stir, aerate for 1 to 2 hours, and test again. Temperature affects the reading — if your water is cooler than 77°F (25°C), the specific gravity will read slightly higher than it actually is. Warm the water to 77°F before taking a final reading, or adjust your target number based on your thermometer.

Common mistakes to avoid

Using table salt or rock salt instead of marine salt mix is the most common error. These products lack the minerals your fish and corals need and may contain additives that harm your tank. Always buy salt labeled for aquariums.

Skipping the 24-hour aeration period is another mistake. Salt needs time to fully dissolve, and aeration helps this process. If you add water to your tank before aeration is complete, the specific gravity will continue to change as the salt finishes dissolving, throwing off your tank's balance.

Not measuring specific gravity is a third pitfall. You cannot know if your water is mixed correctly without a hydrometer or refractometer. Guessing by taste or appearance will lead to water that is too salty or not salty enough, stressing your fish.

Storing mixed saltwater for later use

You can prepare saltwater in advance and store it for up to two weeks in a sealed container away from direct sunlight. Label the container with the date and the specific gravity reading. Before using stored water, check the specific gravity again — it should not have changed, but if it has, aerate the water for a few hours and retest.

Keep the container sealed to prevent evaporation, which would raise the specific gravity over time. Store it in a cool place — heat speeds up algae growth in stored saltwater. If you see algae growth (green or brown discoloration), do not use that water; prepare a fresh batch instead.

Frequently Asked Questions

Can I use bottled drinking water instead of tap water?

Yes, bottled water works and is sometimes better than tap water if your tap water is heavily chlorinated or has high mineral content. Distilled water is the safest choice because it has no minerals or chemicals, though it costs more. Avoid bottled water labeled "mineral water" — it has added minerals that will throw off your specific gravity reading.

What temperature should the water be when I add it to my tank?

The water should be within a few degrees of your tank's temperature. If the mixed water is much cooler or warmer, let it sit in the tank for 15 to 20 minutes before pouring it in, or pour it slowly over a period of a few minutes. A sudden temperature change stresses fish and can crack the tank glass.

Why does my hydrometer reading change between measurements?

Temperature is the most common cause. Specific gravity readings are calibrated for 77°F (25°C); if your water is cooler, the reading will be higher than the actual salinity. Always measure at the same temperature, or use a refractometer, which automatically adjusts for temperature.

How often do I need to make new saltwater?

You make new saltwater only when you do a water change — typically 20 to 30 percent of your tank volume every two to four weeks, depending on your tank's bioload and whether you have corals. A 20-gallon tank might need 4 to 6 gallons of new saltwater per change.

Can I reuse the old water I remove from my tank?

No. Old tank water contains fish waste and decaying food that will foul your tank if you return it. Always discard old water and prepare fresh saltwater for water changes.