how to make saltwater

How to make saltwater from reef salt mix

Have you ever looked at a saltwater tank at a city aquarium, or even a dentist’s office and thought, “I wish I could have one of those!” but don’t even know where to begin? For example, where do they even get the saltwater for that reef tank from?

To make saltwater for fish, mix o.5 cups of reef salt into each gallon of fresh water, purified by reverse osmosis and deionization, to achieve the same specific gravity as natural seawater, 1.025.

saltwater aquarium setup
Want to know how to make saltwater for an aquarium like this?

It’s no mystery, the saltwater from that reef tank came from one of three places:

Three common sources of saltwater for fish and coral aquariums

1. If you live near the ocean, the water may actually be from there

If the location is near the ocean, it is possible, although unlikely, that they actually get their saltwater from the ocean. The ocean is a great place to get natural seawater for a saltwater reef aquarium, but it does have a few drawbacks:

  1. The water near where we live is sometimes less than pristine and may not be suitable for a home reef tank
  2. Natural seawater may contain unwanted parasites or hitchhikers
  3. Harvesting seawater from the ocean can be labor or fuel-intensive and logistically challenging

Many reef aquarium hobbyists who want to eliminate the potential unknown factors associated with harvesting natural seawater, or for the vast majority of us who don’t live near the ocean, some will actually purchase their saltwater from their local fish store.

2. Purified water was purchased pre-mixed from a local fish store

Local fish stores typically have some sophisticated equipment, called a RODI filter, which purifies the water before converting it to saltwater for the reef tank. RODI stands for Reverse Osmosis De-Ionization, which describes the multi-step process this equipment performs to purify the water. RODI essentially filters tap water to make it pure water before the salt is added. The advantages of buying saltwater from a local fish store are that you don’t need to:

  • Purchase expensive equipment like a RODI filter to make the water
  • Perform any maintenance on the equipment
  • Spend any time to mix the saltwater yourself

However, buying saltwater at your local fish store can be expensive and still significant logistical challenges–especially for the aquarium hobbyist with a rather large saltwater reef tank.

3. They learned how to make saltwater for fish and coral aquariums using a reef tank salt mix and water from their tap

The most common way how to make saltwater is to combine a carefully measured amount of reef tank salt mix with water from your tap (perhaps water that has already been purified). Don’t worry, this is super-simple and can be completed in just 5 steps. Here is an infographic to help you remember how to make saltwater for fish in 5 simple steps:

How to make saltwater for aquarium fish

How to make saltwater for fish and fish tanks: A simple step-by-step guide

The process for how to make saltwater for a fish tank is fairly simple and can really be done with just a few simple steps.

Step 1: Buy a high-quality reef aquarium salt mix

The first step in how to make saltwater for fish is to purchase a high-quality reef salt mix. Your local fish store will have several salt brands for you to choose from. If you aren’t sure which mix is best for your needs, I encourage you to check out my reef salt mix product review. These days, I get my salt online. It’s cheaper and I no longer have to worry about these heavy buckets rolling around in my trunk.

how to make saltwater: Step 1

Just be sure NOT to buy the little carton of API-brand “aquarium salt” you might find at Petco or Petsmart. That’s not the same thing. You want to check the package and make sure it is a reef tank or sea salt mix. Instant Ocean is the #1 brand. You can’t go wrong with either the Instant Ocean Sea Salt or Reef Salt.

Step 2: Put on gloves and follow the manufacturer’s instructions (hint: measure out a half cup of salt mix for every gallon of water you will make)

The second step in how to make saltwater for fish is to put on gloves and follow the instructions on the reef salt mix package. Most salt mixes call for about 1/2 cup of reef salt for every gallon of fresh water–so use a measuring cup to measure out the salt precisely. The gloves will protect your hands. The salt is very fine and it will irritate your skin if it rubs on it.

atlas gloves for aquarium

The manufacturers tend to make recommendations on the light side. For example, the salt mix I personally use is Instant Ocean, which calls for 1/2 cup of salt mix for every gallon of water.

But this recipe makes water at a specific gravity of 1.021, which is fine for a fish only tank but is a bit low for a reef tank. To have reef-quality water, you will need to mix in more salt~ approximately 1/2 cup + 1-2 tablespoons per gallon.

Step 2: Measure out a half cup of salt mix for every gallon of water

Scoop 1/2 cup (unless the package tells you differently) into a reef-safe, chemical-free bucket used only for mixing saltwater or performing water changes. If you don’t have gloves yet, check out this review of some popular and inexpensive aquarium gloves.

Step 3: Add purified (RODI if you have it available) or dechlorinated tap water (1 gallon for every 1/2 gallon of salt mix you measured out)

The 3rd step in how to make saltwater for an aquarium fish tank is to add the correspondingly appropriate amount of water to that bucket, based on how much salt you measured out and how much saltwater you need to make. For example, if you added 4 x 1/2 cups (2 cups total) of the salt mix, you want to add 4 gallons of water to the bucket and stir well, while the bucket fills up with water.

Step 3: Fill the bucket with 1 gallon of water for every 1/2 cup of salt mix

Step 4: Aerate the water

The 4th step in how to make saltwater is to use a small air pump and airline to aerate the water. Mixing the salt into the freshwater causes a chemical reaction and it takes some time for the chemical properties, like pH to stabilize. Aerating the water for a short time helps adjust the oxygen/carbon dioxide and pH levels.

Step 4: aerate the saltwater


Step 5: Confirm you made it to the proper concentration by measuring the specific gravity (density) with a hydrometer

The 5th step in how to make saltwater you take before adding the saltwater to your tank is to use a hydrometer to measure the specific gravity of the water in order to confirm the water has the right amount of salt in it.

Step 5: measure the specific gravity (density) to confirm the concentration
use a hydrometer to measure the specific gravity

M0st of the time, our goal as aquarium-owners is to try and recreate a natural reef environment. Natural seawater has a specific gravity of 1.025, so that’s a great density to hit. This is why it is important to read the manufacturer’s instructions and to measure what you created. As I mentioned earlier, the instructions on my favorite bucket of salt make a fairly weak seawater concentration of 1.021. So I always give it a few more tablespoons of salt mix for every gallon of water I make.

How To: Mixing Saltwater for Beginners | Saltwater Tank

Step 6: (optional) Record your specific gravity reading and your water change

It’s never too early to develop the habit of carefully recording your test results and any observations you make. Taking a scientific approach like that will help you develop good habits and troubleshoot if you ever run into any problems. A great tool you can use is a

Use a reef journal aquarium log to track your readings over time to stay disciplined and look for any problems. You can learn more about this and other great aquarium books here.

I didn’t include this in the simple 5-step process because you technically don’t need to complete the step to have made really great aquarium saltwater. However, it is a recommended, optional step.

track calcium levels over time


This process of how to make saltwater from a reef salt mix can get a bit more complicated if you don’t have a high quality, pure water coming from your tap. If there is a lot of chlorine in your municipal water, you may need to use a de-chlorinator to purify your tap water first, or you may need your own RODI system to purify your water prior to use.

However, if the notion of purifying your water terrifies you and is keeping you from diving into the hobby, let me share with you my own experiences.

When I first started out, I lived in a suburb of Philadelphia, with chlorinated municipal water. I mixed the salt with water right from the tap—but I did give the water a few days for the chlorine to dissipate.

I later moved to a different municipality (also in PA), and did the same thing, and finally moved to my current home with well water (some of the time it was even unfiltered).

It certainly would have been better had I used purified water–and officially that is what I recommend, but don’t let that keep you from getting in the game. If you are a beginner in the saltwater aquarium world, start off small, with a nano aquarium.

Try out a fish-only aquarium first with a tried and true species, like captive-bred clownfish, and get some experience mixing the saltwater with tap water salt mix first–before you go crazy with all the extra purification equipment.

Don’t get me wrong–the highest quality water will give you the best results, but depending on the size of your set-up and the purity required by the animals you plan to keep in your tank–all of that complexity and expense may not be warranted, so why not start out slowly–test, observe and see.


This was a simple guide designed to show you how to make saltwater for fish and aquarium corals. Hopefully, you would agree, the process is fairly simple. Is there anything else that would be helpful to know about how to make saltwater? If so, please leave a comment below so that I can know how best to update this article.

Recommendations on what to read next

Learning how to make saltwater for fish or a coral aquarium is an important and simple first step. If you want to learn more about how to build a better saltwater aquarium, check out some of these top articles:

Written by Albert B. Ulrich III–author of The Reef Aquarium Series of books:  The New Saltwater Aquarium Guide, Reef Journal, How to Frag Corals, 107 Tips for the Marine Reef Aquarium.





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