Terrible Advice Tuesdays (T.A.Tues): The Salinity Level That Can Kill?


Terrible Advice Tuesdays (T.A.Tues): If your saltwater tank’s salinity level is under 1.024 specific gravity (32 ppt), then your corals will die.

The rest of the story: Corals are less tolerant of lower salinity levels than fish and most corals will survive with levels as low as 1.020 (26.6 ppt). Granted the corals likely won’t be happy – they may turn brown or bleach – they can survive.

Recommended salinity levels for a reef tank are 1.024 – 1.025 (32 – 33 ppt) and if you are slightly below or above that level (1.022 – 1.027), your tank will be just fine. Of course if your levels are out of the 1.024 – 1.025 range, I do recommend you bring them back in check.

If your salinity level is low, I recommend doing a series of water changes with higher salinity water to bring your salinity back to the recommended levels. For example, if my tank’s salinity was 1.022, then I would do my weekly 5% water changes with saltwater with a salinity level of 1.030. After each water change, I would test the salinity of my tank’s salinity to monitor the salinity changes.

If your salinity level is high, doing water changes with lower salinity water or replacing small amounts of your tank’s volume with straight RODI water will lower your salinity levels.

I DO NOT recommend you mix salt directly into your tank, or even into a high flow area of your sump! Salt crystals can burn corals or fish! Also, chemical reactions are happening as the salt and other chemicals are dissolving in water and you don’t want your livestock in the direct path of these reactions.

Mix your salt in a bucket or mixing container, then do your water change.

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Comments for this article (21)

  • Joe says:

    Great advice Mark, If I find my salinity is low, I will change my auto top off water container to one I have that holds saltwater and let it slowly add the higher salinity water to the tank. I will continue to check the salinity level over the next few days and change back to my ro/di water when I reach the correct level in my tank. Thanks for all your help over the years and I find your Q and A Monday fantastic.

  • Laurie says:

    great advise Mark, and I have just encountered this very problem, I took my salinity checks and found that my readings were 1.028, so I then began doing a water change by slowly adding 5 litres of unsalted RODI water into my tank and letting the water run through my filter system for about half an hour before adding another 5 litres until the reading I obtained was 1.024.
    Tanks again Mark for all the effort you put into your videos and the time spent in answering all our questions.

  • James says:

    I recently had the opposite problem and my salinity was up to 1.032, my corals did not like it at all, it as been a week and I am not sure how many I will loose, How high can corals go before they die?

  • JasPR says:

    Give an inch and a driven,motivated reef enthusiast will take a mile!!! we do indeed need to be sensitive to the needs of our aquarium charges. BUT obsessing on corals as though they are rare hot house plants is an example of one having too much time on their hands! 🙂
    Honestly one of my fun manipulations is get friends into the hobby! I oooo and aaaahh them with the tank presentation and then drop bits of interesting facts about the sea creatures within. Soon they are hooked and I get to set up another aquarium that I don’t have to take care of! LOLs
    But one thing I suggest that those still in the hobby after a year or two do, is to take a dive trip or two to see where their pets live in the wild. There is NOTHING like seeing the place they come from- feeling the temperature changes, the currents, the storm damage and the extreme conditions many coral species live in. HOT afternoon water ( I’ve been exploring shallow coral reefs in 84-86F water and then in 72 F water within hours. I’ve seen them baking in sun at low tide and in other cases, polyps hidden in areas of very poor sunlight. I’ve seen anemone way way too deep to get adequate sunlight ( caribbean’s Saba island) at the base of an atoll in 70 ft of water.
    In short, follow the advise of experts but when you see obsessive compulsive thoughts, recognize them for what they are. A word to the wise—
    Happy New Year to all! JasPR

  • Chris Goddard says:

    I used to use a hydrometer to check my levels and thought my tank was at 1.024. On the off chance I took a water sample to my LFS just for a check to make sure my test kits where ok, but to my suprise my salinity levels were 1.030 checked through a refractometer!! Good job ive been a good boy this year as Santa bought me one and I have now got my levels down to 1.025.

  • Bob says:

    If you use two part solutions to raise your calcium and alkalinity levels, you should monitor salinity more frequently, as they do have an effect on specific gravity.

  • Mark says:

    Hi! I keep a 5 gallon bucket for a 90g reef with 90 pounds of rock less displacement, plus sump to total 108 gallons. One 5 g bucket takes 2 cups of salt and my salinity is a constant 1.025 and hasn’t changed in 10 months, the day the tank was born. A simple measuring cup once you have established the amount of salt will guarantee that your tank will be stable. Remember of course, that if your skimmer overflows into the cup and drain you will have to replace that water with mixed salt water, not a lot though, but it keeps everything in balance. Just my 2 cents.

  • Wade says:

    When my salinity gets a little low I usually add a half gallon or gallon of my premade salt water to my sump and then scale back my ATO a bit to let the water evaporate over a day or so leaving the salt behind and thereby slowing raising the salinity.

  • corocora says:

    Coral Sea salt mixing instructions say to test for salinity with the water temperature at 77 degrees. For accuracy always bring the temperature up to the same level (near there) each time you mix, and using a properly calibrated refractometer is a must. Before I switched to Coral Sea I was mixing and measuring at whatever temperature my RO/DI water happened to be at, and was probably creating inconsistency unknowingly.

  • corocora says:

    Sorry, I meant Red Sea Coral Pro mixing instructions.

  • Mark says:

    Yes. I meant to add that the tank is a constant 78•. Having an aqua controller as well programs the temp along with internal power heads for varied circulation. I got mine off eBay for 1/2 the price of a new one. It measures pH too.

  • Pierre Bouic says:

    JasPR has a good point about what coral or fish for that matter can put up with. A couple of eg.’s when the big rain hits in north Queensland Aust. the Salinity can drop for a couple of months. Corals on the lagoon side of the GBR (Great barrier reef ) experience drops in salt levels all at once, by means of the tidal line of muddy fresh water getting down to 1.019 as soon as the line of tide passes. This can be, for a coral head underwater less than 5 mins dropping from 1.026. So fish as well as corals experience this as well as temp drops very suddenly compared with what we would submit our prized pieces to. I was thinking about this last week as I brought home some new fish to drop them in to their own QT which had a salinity of 1.025, compared to the bag water that was 1.018. I still took 2hrs to bring up the salinity to match the QT water. Maybe nature can allow things like this to happen without shock. I have once dropped fish into water of a higher value without thinking & suffered no trouble, but temp drops I would never play with.

  • sam says:

    When I do a water change n I want to lower my salt level how much lower should the water I’m adding be

  • Please provide peer reviewed sources for your assertion that anything lower than 1.020 (26.6 ppt) will cause coral death.

    Thiel, Albert J. 2003. Parasites and Low Salinity
    Thiel indicates that no long term ill-effects could be found at levels as low as 1.017, with very few exceptions, as long as Calcium and Alkalinity levels are maintained.

    Pete Giwojna also studied and published hyposalinity information that indicated corals were safe at 1.017 and lower. The information was gathered as part of his research regarding seahorses.

    There are numerous other sources that can be cited. Kathrine Chartrand did a lot of work studying Siderastraea in Florida Bay in 2006 and 2007, presented a thesis and several papers on the subject, also peer reviewed.

    I am still, after all of these years, stunned at the amount of bad information that is passed around in this hobby by “experts” as off-hand fact.

  • BeanAnimal…note that the article said “most corals” not “all corals”.

  • Mark,

    Most of the research I have read indicates differently and only shows a few corals that struggle below 1.017. While keeping specific gravity in the range you indicate is certainly NOT bad advice, the notion that keeping it lower will kill “most corals” is poor information and somewhat contrary to the intent of this column.

    All in all, good advice but the lower limit information needs to be clarified.

  • Rafael Gomez says:

    I have a 60g rimless acrylic tank and my salinity is at 1.021 I changed 15g recently all with salinity at 1.030 and it didn’t really make it different salinity only went up by one, I have a frogspawn coral in there that’s still doing great but want to get the salinity fixed before he dies out any advice?

  • Fish- Head says:

    I live in the Caribbean/Bahamas and the Salt water that I take directly out of the ocean measures at about 1.032 S. Gravity (45 Salinity).
    I have had a Salt water aquarium for 20 years now and use only Pure Saltwater from the Ocean (no mixing salt in or anything like that, pure salt water).
    Now I must admit that I have had a lot of different Parasites (Flukes, Ick, Etc….) over the years. I catch all of the fish directly from the Ocean Reef, so there is no need for Quarantine, right?
    I have read about 100 reviews and everyone says to keep the Aquarium between 1.023 & 1.026. But that is a big deference then the Ocean Salt Water by a long shot.
    Should I bring the Ocean water down to the suggested levels or should I leave it at the same amount that I am getting the Juvenile Fish from. It would seem that if I change the Salinity level that the fish have been living in their entire lives, the fish would be uncomfortable and possibly die.
    How did everyone come up with this perfect number 1.025, when the Ocean itself is about 1.032?
    Someone help me here, before any of my fish become sick or possible die. Which salinity figure should I go with?

  • That is a very good point fish-head left. I’ve been keeping fish since before the 80s. I’m no expert, and I’ve personally done many many many brackisk to Marine “what ifs” and expermints. from what I have observed …. may the many who passed Rip! if its going to thrive, stick with what many years of evolution has allowed the programming to except! if it ain’t broke don’t fix, and most of all these are living creatures like you and I. they will talk, walk, and express themselves in a language we are only beginning to just understand. Watch, learn, learn , learn, and be observant to what’s being said. now for Marine Biologist with DNA and clone education, send me a link please. I’m more curious what we as caretakers of the planet can do in the area of genitic splicing and engineering for our “animal-plants” to survive and proliferate in an ocean that will become warmer, less saline to very saline and toxic as time, technology, and extinction of these creatures moves forward-Williamssteven1966@yahoo.com

  • I’m mixing water for my red mangroves that live in brackish water. Ive used 1/8 and 1/4 teaspoon Ocean salt per gallon of water and still each time it goes up to 1.025. How do you get it lower? These mangroves I bought the salinty in the water wasnt higher than 1.010

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