Our Privacy/Cookie Policy contains detailed information about the types of cookies & related technology on our site, and some ways to opt out. By using the site, you agree to the uses of cookies and other technology as outlined in our Policy, and to our Terms of Use.


How to Correct Marine Aquarium Salinity

By Eric Mohrman

Jupiterimages/Photos.com/Getty Images

Maintaining a saltwater or marine aquarium takes a bit more diligence than maintaining a freshwater tank. Both tanks should generally have water between 75 and 80 degrees Fahrenheit. However, with a marine aquarium you have to keep an eye on several factors of the water's chemistry. Pick up a testing kit to monitor the water's pH, nitrate, nitrite, ammonia and phosphate levels. Also, watch the water's gravity, or temperature-dependent salinity reading. While most marine life can handle a range of salt content, they need it to remain consistent at whatever level they become accustomed to. Make prompt corrections when necessary.

Step 1

Follow the manufacturer's instructions for the particular salt mix you use when setting up your marine aquarium, and whenever you're adding new water. Use a hydrometer, refractometer or specialty salt monitor to measure salinity. Maintain a constant gravity reading between 1.021 and 1.024. Do not allow changes greater than 0.001 to go uncorrected,

Step 2

Siphon up to 10 percent of tank water at a time and add freshwater in its place to lower the salinity and gravity to the desired levels. Added water should be the same temperature as the aquarium water.

Step 3

Remove up to 10 percent of aquarium water at a time and replace it with salt water to raise the salinity and gravity to the desired levels. Mix the salt water in a large bucket according to the directions and bring it to the same temperature as the tank water before adding it.

Step 4

Change the marine aquarium's gravity by no more than 0.001 in a day. A greater change in either direction can cause shock or death to aquarium inhabitants. Correct salinity gradually over the course of a few days if necessary.

Items you will need

  • Salt mix
  • Hydrometer, refractometer or specialty salt monitor
  • Bucket


  • 💡 Maintain a gravity log to track your marine aquarium's salinity. It's not as important where in the 1.021 to 1.024 range it falls as it is that the level is kept constant; variations in salinity can shock tank inhabitants and even kill more sensitive ones.
  • 💡 Remember to use freshwater when replenishing dropping water levels every day or so to maintain constant salinity. The aquarium's water level drops because of evaporation, and only the water itself evaporates; the salt content remains in the tank.

Photo Credits

  • Jupiterimages/Photos.com/Getty Images


Eric Mohrman has been a freelance writer since 2007, focusing on travel, food and lifestyle stories. His creative writing is also widely published. He lives in Orlando, Florida.