A Step-by-Step Guide to Mixing Saltwater

A popular synthetic sea salt choice

A popular synthetic sea salt choice

The foundation of any saltwater aquarium is, of course, the salt water itself, and through your time in this wonderful hobby, you’ll be mixing up plenty of it! You’ll need it when you first fill your aquarium and quarantine tank, whenever you perform a routine water change, any time you’re adding and acclimating new specimens, and for any number of unforeseen circumstances that might crop up along the way. It’s always wise to have plenty of extra salt water on hand.

Now, unless you happen to live on the coast and have access to a limitless source of clean natural sea water (which is actually more problematic than it might seem), you’ll need to make your own salt water using fresh water and a synthetic sea salt mix. Here’s how it’s done:

What you’ll need:

Step 1

Fill your bucket or bin with clean fresh water. Remember, the salt mix will displace some of the water, so don’t fill the container completely to the top. Ideally, you’ll want to use water that has been purified through reverse-osmosis and/or deionization. Dechlorinated/dechloraminated tap water can be used for a fish-only system if you have excellent source water, but RO/DI water is still the safest bet.

Step 2

Take note of the manufacturer’s directions for a salt mix per gallon specification and begin stirring in sea salt slowly and methodically, testing the water with your hydrometer or refractometer to monitor the specific gravity (an indirect measurement of salinity).

For our purposes, we’ll assume natural sea water around the tropical coral reefs has a specific gravity of about 1.025 (salinity of 35ppt). This is a good target, especially if you have or plan to keep any corals or other invertebrates in your system. Fish-only systems can be maintained at a slightly lower specific gravity if desired, as low as 1.020 or anywhere in between.

Keep track of how much salt you’re adding throughout the process. That way, you’ll have a pretty good idea how much you’ll need to add the next time you’re mixing a batch of salt water.

Step 3

Once the desired specific gravity is reached, place the powerhead in the container and plug it in to provide circulation. Try to position the powerhead so its output creates some turbulence at the surface of the water in order to maximize gas exchange. Then, put in the submersible heater with the thermostat adjusted to the same temperature as your aquarium.

Step 4

Let the water sit, circulating and heating, at least overnight before using it. This will allow enough time for the salt to dissolve completely and the desired temperature to be reached.

Step 5

The next day, test the specific gravity again to ensure it’s still at the desired level. Adjust it up or down as necessary by adding salt or fresh water. Also, check the water temperature and adjust as needed.

If the levels are where you want them, you can go ahead and use the saltwater as needed to fill your aquarium, replace water following a water change, etc.

When initially filling a new aquarium, and before any livestock or live substrates (e.g., live rock or live sand) have been added, saltwater can be mixed in the tank itself. After that, it must always be mixed in a separate container.


When water evaporates from a saltwater tank, all the dissolved solids in it (read: the salt) get left behind. Thus, top offs to compensate for evaporation must be made with purified fresh water, not salt water.


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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. Hi Jeff,
    Thank you for the wealth of CURRENT information, it has been a lifesaver for me and my fish when I can’t figure out what to do.
    Re: mixing salt. Sometimes I mix the salt water but can’t get to my water changes for a day or two. Is anything “bad” happening to my water over this period. Are things starting to grow in the bucket? Sometimes I leave the power head in, sometimes not.

    • Jeff Kurtz says

      Hi Kathy! Thanks so much for your kind words! It’s no problem whatsoever to mix up a batch of salt water and then wait a few days to do the actual water change. In fact, I often wait several days to a week or more before actually using newly mixed salt water. Nothing bad is happening to it in the meantime. Just be sure to keep the water aerated and heated to the desired temperature.

      Also, it’s a good to keep the container covered to minimize evaporation, which will increase the specific gravity of the water. If some evaporation does occur, no worries though. Just remember to add fresh water to bring the SG back to the desired level (testing with your hydrometer, of course) before doing your water change.

  2. hey man your tips are really useful but i want to know if i have a 90 gallon tank how many days would it take for me to fill my tank up the first time since u said u need to wait for the water to sit overnight


    • Thanks Michael! I wouldn’t worry too much about aging the salt water for your initial tank fill (before any livestock is added). That’s more of a concern for performing routine water changes once the tank is up and running. I failed to make that clear in the post.

  3. I bought salt water a few weeks ago I put it in the shed in a container will I still be able to use it

    • Hi Jane! That shouldn’t be a problem. Just be sure to aerate the water for a day or so and heat it to the appropriate temperature before using it in your tank.

  4. Hi I’m mixing up salt water, my tank is running at 26c and has a salinity of 1.025. Iv just mixed up a drum of 75litres and its salinity is 1.025 but temp is 11’c will this change much once I heat it up to 26’c

    • Jeff Kurtz says

      Hi Kevin! Specific gravity is temperature-dependent, so you’ll want to heat the water to the desired temperature and then adjust the SG from there. Thanks for your question!

  5. Hello Mr. Kurtz,

    My name is Marco and I am a senior at Minnetonka High School. I am writing on behalf of myself and my research partner in hopes that you could provide us with some quick advice on some difficulties we have encountered. For our research, we hope to investigate the recovery of Aiptasia Pallida, a marine anemone, under different temperature conditions. However, in setting up our marine tanks(four ten gallon tanks), we made mistakes. In hindsight, our mistakes seem silly, but we would like your opinion on how to address them. Here are our main errors:
    When mixing in the salt mixture, we used conditioned tap water(but not DI water). We now have access to DI water.
    We mixed the water in our tanks instead of in a separate bucket. We now have good buckets available.
    The buckets we used to gather the water were not high grade plastic. Our new buckets are safe plastic.
    To mix the saltwater we did not use a pump, but instead vigorously mixed each batch until it appeared to dissolve. We do not have a pump but could get one, is it very necessary?
    With all of these errors, our first Aiptasia that we added to the tanks died overnight. Moving forward, we were planning on a large water change(50%) with saltwater mixed correctly. Do you think this would suffice, or should we drain all of the water and start again? Any advice is welcome.

    Best Regards,
    Marco Conati

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