5 Tips for Maintaining Stable Salinity in a Saltwater Tank

A refractometer or hydrometer are common tools used to test salinity

A refractometer or hydrometer are common tools used to test salinity

While salinity and other water parameters are relatively constant on the natural coral reefs, they’re highly prone to undesirable fluctuation in the closed system of a marine aquarium. As conscientious hobbyists who want to provide the most naturalistic environment possible for the sensitive fish and invertebrates in our care, one of our primary responsibilities is to maintain stable water-chemistry values in the face of various influences that tend to promote instability.

When it comes to maintaining stable salinity (or specific gravity, depending on what you’re actually measuring) in a saltwater system, you’ll find there are several factors at work to undermine your efforts. Here’s how to counteract them:

#1: Top off for evaporation with fresh water only

Remember: salt and other dissolved solids do not evaporate. When evaporation occurs in your aquarium, the salinity actually increases as the water volume decreases. Thus, if you top off your tank with salt water to compensate for evaporation, the salinity will climb because now you have more dissolved salt than you began with. On the other hand, replacing the evaporated water with an equal volume of purified fresh water will keep the salinity right where you want it.

A key phrase in that last sentence is “an equal volume of purified fresh water.” For the sake of stability, it’s important that you replace no more or less water than was lost to evaporation. To ensure that you’re adding the correct volume at each top off, it’s helpful to mark the desired water level on your aquarium or sump in some manner. For example, I simply place a piece of masking tape on the sump of each of my systems at the water level I want to maintain.

Also, try not to allow too much time to elapse between top offs. The longer you wait, the steeper the fluctuation in salinity will be. Freshwater top offs should be considered a daily chore.

#2: Match salinity when mixing new salt water

Of course, any replacement salt water you use for water changes must be of the exact same salinity as the dirty water you’re removing. But don’t rely on the measurement you get immediately after mixing new salt water. Let the water sit at least overnight while it’s being heated (to match the temperature in your tank) and aerated, then test it again before using it. Sometimes you’ll get a slightly different reading after the water has had a chance to mix and stabilize, in which case you may need to make slight adjustments by adding either more salt or fresh water before using it in your tank.

#3: Make one-to-one water changes

Just as you need to compensate for evaporation by adding an equal volume of purified fresh water, any time you perform a routine water change, it’s critical to replace no more or less salt water than you removed. To achieve this, you can use that same mark on your aquarium or sump as a reference for how much salt water to replace (assuming you’ve already topped off the tank that day) or you can simply keep track of how many buckets, etc., of water you remove and replace the same amount. If you use the latter technique, you must also be sure to fill the bucket or other vessel to the exact same level each time.

#4: Compensate for salt creep

One of the most visible factors influencing salinity is salt creep—that crusty salt buildup that occurs on any surface exposed to air and saltwater spray, such as the top edge of the tank, the cover, power cords emerging from the aquarium, etc. Over time, this loss of salt can lower the salinity, so you may occasionally need to compensate by adding small quantities of sea salt (dissolved in aquarium water and added very gradually) to your system.

#5: Don’t forget the small stuff

In addition to freshwater top offs, water changes, and salt creep, there are numerous smaller factors that we don’t tend to give much thought but can still have an effect on salinity over time. For example:

  • Whenever you acclimate a new specimen, some salt water is necessarily removed from your system. If you forget about this and later top off the tank with fresh water, you’ll end up lowering the salinity in the process, if only to a small degree.
  • If you get into reefkeeping, a similar example would be bagging up coral frags to sell to your local dealer or fellow aquarist. This often involves the removal of small amounts of aquarium water that would need to be replaced with an equal volume of salt water.
  • Your protein skimmer can even have an effect on the salinity level. Each time you empty your collection cup, you lose a small amount of salt.

None of these minor factors taken singly is going to cause a sudden, precipitous shift in salinity, but over time, they can have an impact. Regular testing with a refractometer or hydrometer—a routine maintenance chore that should be conducted on at least a weekly basis—will reveal any shift away from the desired salinity/specific gravity level, so you can react promptly to correct the problem before it becomes too pronounced.


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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. This is good to know! I’m planning on starting an aquarium in the next few weeks, so I’m doing all the research I can. Water treatment sounds a bit overwhelming at first, but this gave me some direction on how to maintain it. Thank you!

    • Jeff Kurtz says

      It’s our pleasure, Stephanie! Good luck with your new tank, and don’t let it overwhelm you. Take it one step at a time, and keep in mind that only bad things happen quickly in marine aquariums.

      • This article says when doing a water change make sure the salinity is the same as the dirty water…… my question is what if the salinity in the tank is a little high or low?

        • Hi Andrew! If the salinity in your tank is high or low, you can adjust it in a variety of ways. For example, if you need to raise it, you can dissolve a small amount of 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. If you need to raise the specific gravity substantially, you’d want to do this in several doses over the course of a few days.

          Another option for raising the level is to replace dirty water removed during water changes with clean salt water mixed to a slightly higher-than-normal specific gravity or to top off water lost to evaporation with saltwater rather than fresh until the desired specific gravity is reached.

          If the specific gravity of your tank is too high, you can either remove small amounts of salt water and replace them with fresh water or use water mixed to a slightly lower-than-normal specific gravity for water changes.

          Of course, whatever the circumstance or approach you take to remedy it, it’s critical to change the specific gravity very gradually to avoid shocking your livestock.

  2. How to do regarding salt creep?

    Above article said ” Over time, this loss of salt can lower the salinity, so you may occasionally need to compensate by adding small quantities of sea salt (dissolved in aquarium water and added very gradually) to your system.”

    can I just drop a scoop of sea salt directly into the tank? small spoon, then measure it..
    wait one day., then each other day do the same
    and keep adding until the measurement hits 1.024?

    originally salinity was 1.024
    right now it is measuring 1.021


    • Jeff Kurtz says

      Hi Chip! I wouldn’t drop the salt directly in the tank. I would recommend that you fill a cup or other small container with aquarium water, dissolve the sea salt in it, and then drip it slowly into a high-flow area of the tank. Beyond that, you’re right on target with the idea of doing this over the course of several days until you get back to your desired salinity.

  3. Hi Jeff,

    Cycle is almost done but I’ve lost about a gallon of water through evaporation. Should I just add the spare saltwater (and do a salinity test) while there’s no livestock to top it up, or get the freshwater instead? The tank is 55g so it’s not a huge amount to top-up and I don’t change it’ll increase the salinity too much. Thoughts?

    • Hi Wayne! You’ll definitely want to use fresh water to compensate for evaporation. Otherwise, your specific gravity will keep rising. Remember, the salt and other stuff dissolved in the water doesn’t evaporate.

  4. Hello,

    I found out my salinity dropped below 1.025 was told they have to figure out how to bring it back up. This may sound crazy but for some odd reason the strips I used was not for salinity testing. A little ticked off but I recently purchased a wrasse the day before I found out about the salinity dropped below. What can I do to bring levels up. I am currently waiting on the meter for reading salinity.

    • Hi Katrice! You’ll definitely need to use a hydrometer or refractometer to test your salinity/specific gravity. Once you have it and can verify exactly where your reading is, you can adjust it up or down if necessary. If you need to raise the level, you’ll want to do so very gradually. I usually dissolve a small quantity of sea salt in a beaker filled with aquarium water and drip it slowly into my sump. If the level is significantly lower than it should be, I’ll do this several times over the course of a few days until the desired level is reached.

      • Jeff…thanks so much for your quick response. If you don’t mind I have a few other questions. I asked the LFS if I needed to do another water change and was advised no. Long story short, I was told that they needed to figure out how to bring up the salinity. They sent out a guy to take out 15 gallons and replace with salt and fresh water but that was something I could have done myself. Just did a water chane on Saturday. It made absolutely no sense. So I am charged for a maintenance fee which could have been avoided. So hopefully the wardley wrasse gets better because it was extremely stressed. Is there anything you suggest I can do to keep the wrasse from dieing? Since they guy came in today to up the salinity I’m still afraid of loosing the wrasse I just purchased. Can I send you a video of what the wrasse is doing? Just to get your opinion?

  5. My Salinity level is reading 40 & Specific Gravity is 1.030 on Instant Ocean Hydrometer. What do I need to do?
    Also, not sure where these levels are suppose to be and what do I need to do to correct these readings?
    Amonia level 0
    Phosphate 10.0
    Nitrate 10

  6. Jeff Kurtz says

    Hi Patricia! You’ll want to gradually bring your specific gravity down closer to a natural level (around 1.025). The link below will take you to a post with suggestions on how to achieve this.


    As far as the other readings are concerned, 0 ammonia is good, and your nitrate is okay depending on what you’re keeping. The phosphate level, however, would be extremely high if accurate. Perhaps the decimal is misplaced? Ideally, you don’t want to exceed .03ppm.

    To get a sense of where your phosphate–and all your other critical parameters–should be, check out this article by Randy Holmes-Farley. I think you’ll find it extremely helpful!


  7. Hello. i need help the salinity of my tank drops to nearly 1.020. What to do? to make my tank again normal?

    • Jeff Kurtz says

      Can you give us a little more information on how you’re mixing saltwater for water changes and compensating for evaporation?

  8. I have a 30 litre salt water tank, I’m waiting for it to cycle so I can put live stock in, but it needs topping up with water, how can I add tap water without ruining the cycle that I’m trying to achieve, at the moment I’m waiting for the ammonia to drop its on the light green shade?

    • Jeff Kurtz says

      Hi Natalie! Topping up your tank with fresh water to compensate for evaporation won’t disrupt the cycling process at all. Just be sure the fresh water you use has either been purified through RO/DI or at least treated with a product to neutralize chlorine/chloramine.

  9. Thank you for your post. This is excellent information. It is amazing and wonderful to visit your site.

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