How To Make Saltwater For An Aquarium (Guide)

How To Make Saltwater For An Aquarium (Guide)


You have finally decided that you are going to start your own saltwater aquarium. There are a lot of supplies that you will need in order to create an environment that will allow your saltwater fish friends to thrive. The most basic component of these supplies is your water and almost no one has clean saltwater in their backyards so how do we get some for our saltwater aquariums? You get some sea salt mix, freshwater, a trashcan and start mixing it together. Of course, there are a few things to consider before you dive in and start mixing salt and water for your fish so let’s discuss them.

Equipment needed for making saltwater

  • Measuring cup for the salt mixture
  • A quality salt mix that can be bought at your local pet store or at an online retailer
  • Aquarium heater to ensure that the water will be closer to ready to add to the main aquarium when you are done mixing
  • Powerhead to help aerate the water during the mixing process
  • Water (bought from the local pet  store,  treated at home with RODI system or another treatment process)
  • Purchase a tub to mix water and salt aim for something that is at least the same size as the aquarium that you are trying to fill. A brute trash can is one of the best inexpensive choices you can make and have a variety of sizes for you to choose from Make sure you get a lid! You should always clean containers before you try to use them for anything aquarium-related. However, if you are only trying to keep up with your water changes a smaller size will do just fine.
  • Optional RODI to treat your tap water at home rather than buying clean water every time. Reef Tank Resource has a great article discussing how you can treat your own tap water.

How to make saltwater for your aquarium (Step by Step)

You should always refer to the instructions on your specific salt mix to ensure that you are mixing that particular salt correctly as they can differ between manufacturers.

  1. First things first, you need the salt that will make your treated water habitable for a saltwater ecosystem. You can purchase salt online or at your local pet store but make sure that you are purchasing the correct salt for your tank.
  2. The salt can irritate your skin if you are not careful so it is always best to put some sort of gloves on for protection.  The salt mixes can generally be measured out with half a cup of reef salt per gallon of treated water. As you might assume, it is important to watch your measurements while creating your saltwater as your fish are reliant on the mix being up to their standards to thrive. However, depending on the type of tank you are curating, you might need to change the measurements of your mix. For instance, 
  3. Scoop the salt into the bucket you plan to mix and add a gallon of water per half-cup of salt that you placed in the container.  A 30-gallon tank will need 30 gallons of fresh clean water and 15 half-cup scoops of salt mix. 
  4. The next step is to aerate the mixture so that the water quality (pH levels, oxygen, Co2 levels) can stabilize and ensure that you are introducing the appropriate water into your tank’s system. 
  5. The final step of preparation is ensuring that the quality of your water is ready for your reef tank’s ecosystem. You will want to use a hydrometer to measure the specific gravity of the water mixture you created. You are trying to hit around 1.025 specific gravity. If your numbers are off, there is a good possibility that there were manufacturer specific instructions that you missed incorporating into your mixing process. Your specific gravity requirements might also be different depending on the type of saltwater aquarium setup you have. Be sure to read all instructions before you begin mixing! You don’t want to waste your mix or your time. You can also measure the salinity of your water with a refractometer.

How much sea salt per gallon for a saltwater aquarium?

It depends on the type of reef tank ecosystem you are curating and the type of salt that you are mixing in so it is important to note the specific manufacturer instruction whenever you are using a new salt mix.  However, as a general rule, you can use a half cup of salt mix per 1 gallon of clean water or 2 and a half cups of salt mix per 5 gallons in case you want a larger multiple.

In any event, it is always important to double-check your salinity levels with a refractometer to ensure that you are introducing water that is safe for your saltwater fish. 

Best salt mix for a reef tank

Instant Ocean is a common salt mix choice for a reef tank and is one of the most affordable sea salt mixes on the market right now.

You can also try other popular brands like Tropic Marin Pro which is found to be a fast mixing salt that does well when stored for long periods of time.

ESV Seawater System is also worth looking into. The mix works fast and helps fight against evaporation due to its sectioned-off components.

Reef Salt Comparison Chart

The following data was compiled by the guys at in a video discussing the best salt reef mix. These guys provide a lot of great info on reefing and I suggest following them for more info.

Links in the table below go to the Bulk Reef Supply website where you can buy any of these salts. I get no commision from these purchases but I feel it’s important for me to refer you to them due to the value they provided that I’m sharing with you.

Salt Type5gal @35ppt (weight)Total WeightTotal Gallons @35pptCostCost/GallonCost/Month (100 Gallons)
Instant Ocean801.0g22345.5g139.5gal$54.99$0.39/gal$15.60
Instant Ocean Reef Crystals804.0g20228.5g125.8gal$62.99$0.50/gal$20.00
HW Marinemix755.0g19524.0g129.3gal$69.99$0.54/gal$21.60
Red Sea Salt746.5g22276.5g149.2gal$72.99$0.49/gal$19.60
Red Sea Coral Pro Salt739.5g22178.0g149.9gal$72.99$0.49/gal$19.60
Brightwell Aquatics Neomarine684.0g20533.0g150.1gal$73.99$0.49/gal$19.60
Tropic Marin Classic Sea Salt Mix734.0g25056.5g170.7gal$83.72$0.49/gal$19.60
Tropic Marin Pro Reef Salt Mix722.0g25094.5g173.8gal$95.37$0.55/gal$22.00

How long can you store pre-mixed aquarium saltwater?

If you are like me, you will want to make your saltwater in bulk to save yourself time in the long run. However, to do that you need to have the proper container to store your saltwater. You want to ensure that the container that you are mixing your saltwater in is safe for aquarium use and will not leach any chemicals into the water. You will want to have a container that can be installed with a lid too. You can go with a trash can if you are looking to make large quantities of saltwater or a smaller (aquarium safe plastic) storage bin with a lid. 

Storage will depend largely on the climate in which your bin is stored but if the lid is secured in a dark cool place evaporation should be minimal. A lot of evaporation will mess with the overall salinity of your water and become potentially harmful to your fish if you aren’t careful. Always double-check your water before adding it to your saltwater reef tank to make sure that the water is safe for use. 

As long as the salt is high quality and your stored your mixture properly, you should be able to store the water for the foreseeable future. Of course, you shouldn’t mass produce water with several tubs so you don’t have to make water ever again. That is just asking for something to go wrong during your future water changes. 

Hydrometer vs Refractometer

It doesn’t matter if you use a hydrometer or refractometer in determining whether your saltwater is saltwater aquarium ready but you should note that the two instruments measure two different things.

A hydrometer will measure the specific gravity of water samples that you place in it. If you are paying attention that isn’t exactly salinity which is what we are supposed to be measuring. Salinity and specific gravity are related as the specific gravity is measuring the density of the water which increases as salinity increases.

A refractometer on the other hand directly measures salinity as we discuss in the section. It is important to note that you are looking for different measurements depending on the tool you are using. 

Whether you choose the hydrometer or refractometer, just remember that you measuring different details that add up to the same result. A hydrometer is less expensive and also less accurate. Refractometers are preferred for their precision. The fish in your aquarium don’t care which tool you use to get the results you need for them.

How to use a refractometer

The refractometer is an easy instrument to use to measure the salinity of your water. All you need to do is drop some drops of your saltwater onto the lens of the refractometer and wait a few minutes for the water to stabilize. After about a minute look through the lens to see where you are at and if you are higher or lower than 35 PPT, you need to add more water or add more salt respectively.

There are more expensive versions of this tool that will do most of the work for you and display the information on a digital screen. Options like the Milwaukee Digital Refractometer will help give you accurate readings in no time at all.

Analog Refractometer

Digital Refractometer


Using natural seawater (Pros & Cons)

Of course, if you live near the ocean, your first thought might be to grab a few buckets, drive to the beach, and get your saltwater straight from the source. However, as much sense, as this plan makes on the surface, there are some issues with using untreated salt water from the ocean.  

As easy and convenient it might seem at first to grab your water from the ocean, it can be a challenge based on how far you live from the beach. Also, the water is can be filled with all sorts of potential nasties that will hurt your fish more than help them. 


  • Potentially convenient 
  • Potentially cheap


  • Unwanted hitchhikers
  • Water quality doesn’t meet the standards your fish need to thrive
  • Potentially more expensive

Overall, seawater poses more hazards than it does any real convenience. It might be cool to say to your friends that your water comes straight from the sea but unless your fish all came from the same trip, it might be hard to justify using water that could be carrying some deadly hitchhikers. It honestly is easier to treat your own tap water and create your own saltwater with aquarium salt.

Final Thoughts

Whether you are starting with your first saltwater aquarium or you are thinking about making your water change routine a little more systematic, we hope that you were able to adopt some useful tips for your own aquarium setup. Remember to take care when deciding what aquarium salt you choosing as a cheaper version might not give you the results you are looking for and might prevent you from storing your water for extended periods of time. You want to be able to have that supply for when you need a quick water change!

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