Keeping Salinity in the Safe Zone

Marine creatures thrive in a stable environment. When salinity drifts too high or too low, fish become stressed and corals may suffer. The typical range for a saltwater tank is around 33 to 35 parts per thousand (ppt). Periodic top-offs and water changes keep your tank close to that range, but sudden evaporation or over-dilution can create rapid swings. This calculator tells you how much water to remove or add, along with how much salt to mix when raising salinity, so you can maintain a healthy aquarium.

How the Calculation Works

The amount of water you must change depends on the difference between your current salinity and your target. The basic equation is:

Here $V$ is the tank volume, $\mathrm{S\_t}$ is the target salinity, and $\mathrm{S\_c}$ is the current salinity. The resulting $\mathrm{\Delta V}$ represents the volume of water to replace. If the target is higher, you’ll add saline water or mix salt; if it’s lower, you’ll remove salty water and replace it with fresh.

Units and Quick Copy

Select gallons or liters depending on your setup—the calculator converts automatically. After calculating, tap the Copy Result button to keep the adjustment instructions handy when mixing water.

Example

Imagine a 50-gallon tank currently at 31 ppt. Your target is 34 ppt. The difference is 3 ppt, so:

So you would remove about 4.4 gallons of water and replace it with fully mixed saltwater at the target salinity.

Practical Tips

Always mix your new saltwater separately and check its salinity before adding it to the tank. When lowering salinity, take care not to shock sensitive livestock—remove and replace water slowly over several hours. Keep a log of your adjustments so you can refine your routine and avoid large swings in the future.

Conclusion

Balancing salinity is a key part of marine aquarium care. With this calculator, you can respond quickly when levels drift, ensuring your fish and corals stay comfortable.