Salty Q&A: How Do You Make Small Salinity Adjustments?

When you realize the salinity in your aquarium is off, making slow adjustments is important

When you realize the salinity in your aquarium is off, making slow adjustments is important

Saltwater Smarts visitor Andrew had the following question about our post titled “5 Tips for Maintaining Stable Salinity in a Saltwater Tank.” Since we get similar questions quite frequently, we thought it might be helpful to elaborate on our response in today’s post:


This article says, when doing a water change, make sure the salinity is the same as [that of] the dirty water. My question is, what if the salinity in the tank is a little high or low?” – Submitted by Andrew


If the salinity (hereafter expressed as specific gravity) in your tank is slightly off, you can adjust it up or down in a variety of ways. Let’s look at each scenario separately, keeping in mind that each remedy may need to be repeated several times to restore the desired level:

If you need to raise your specific gravity, your options are to:
  1. Dissolve a small amount of sea salt in aquarium water (e.g., in a cup or beaker) and drip the mixture slowly into your sump or a high-flow area of the tank. This is best done in several doses over the course of a few hours or even a day or more, depending on how far the level must be raised.
  2. When performing water changes, replace the dirty water that you remove with clean salt water mixed to a slightly higher-than-normal specific gravity.
  3. Top off water lost to evaporation with salt water rather than fresh water until the desired specific gravity is reached.
If you need to lower your specific gravity:
  1. Remove small amounts of salt water and replace them with equal volumes of purified fresh water.
  2. During water changes, mix your replacement water to a slightly lower-than-normal specific gravity.
  3. Gradually raise the freshwater top-off level in your sump (if practical). Be sure to mark the new level each time (e.g., with masking tape) so the specific gravity doesn’t bounce around.

Of course, whatever the circumstance or approach you take to correct it, keep in mind that this is not a process to be rushed. It’s critical to change the specific gravity very gradually—testing repeatedly as you go with a hydrometer or refractometer—to avoid shocking your livestock. Remember that altering the specific gravity too abruptly or precipitously is much more stressful to fish and invertebrates than maintaining a stable level that is somewhat higher or lower than the ideal.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. I usually just top it off with fresh RODI before doing a change. I have the level marked in my sump, and that helps a lot.

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