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 0.5 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.
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:
- The water near where we live is sometimes less than pristine and may not be suitable for a home reef tank
- Natural seawater may contain unwanted parasites or hitchhikers
- 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 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.
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)
Before handling salt mix, put on gloves to protect your skin from irritation—the salt is very fine and can be abrasive. For recommendations, see this review of popular and inexpensive aquarium gloves.
Next, 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. Scoop the salt mix into a reef-safe, chemical-free bucket used only for mixing saltwater or performing water changes.
Why do manufacturer instructions often target lower specific gravity? Many salt mix manufacturers base their directions on a lower specific gravity (often around 1.020–1.021) to accommodate a wide range of livestock, reduce risk of overdosing, and avoid liability. This level is generally safe for fish-only tanks but is lower than the natural seawater standard (1.025) preferred for most reef tanks.
Risks of blindly following package directions: If you follow the package exactly, you may end up with water that is too dilute for corals and invertebrates, which can lead to stress, impaired growth, or disease susceptibility. On the other hand, adding too much salt can cause osmotic stress, dehydration, or even death in sensitive livestock.
How to adjust for different tank types:
- Fish-only tanks: 1.020–1.022 specific gravity is generally safe and can slightly reduce disease risk for fish.
- Mixed reef tanks (fish, soft corals, LPS): Aim for 1.024–1.025 to better match natural seawater and support coral health.
- SPS/LPS-dominant reefs: 1.025–1.026 is ideal for sensitive stony corals and invertebrates.
Consequences of over- or under-dosing salt mix: Too little salt can cause osmotic stress, poor coral calcification, and increased disease risk. Too much salt can lead to dehydration, stress, and even livestock loss. Always measure carefully and adjust based on your livestock’s needs.
Troubleshooting tip: If your specific gravity is too low after mixing, add small amounts of salt mix, dissolve thoroughly, and retest. If it’s too high, add purified water in small increments, mix, and retest. Make changes gradually—consistency is more important than chasing an exact number, and abrupt corrections can be stressful for your tank’s inhabitants.
Step 3: Add purified (RODI if you have it available) or dechlorinated tap water (1 gallon for every 1/2 cup of salt mix you measured out)
The 3rd step in how to make saltwater for an aquarium fish tank is to add the appropriate amount of water to your 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.
RODI (Reverse Osmosis De-Ionization) systems remove chlorine, chloramine, heavy metals, phosphates, nitrates, silicates, and other impurities from tap water. These contaminants can fuel nuisance algae, harm sensitive corals and invertebrates, or accumulate over time, leading to long-term tank problems.
Trade-offs:
- RODI water: Highest purity, best for reef tanks, but requires investment in equipment or regular purchases from a store.
- Dechlorinated tap water: More convenient and inexpensive, but may still contain minerals, phosphates, or other contaminants that can cause algae or stress sensitive livestock.
- Untreated tap water: Not recommended for reef tanks; may be acceptable for hardy fish-only setups or temporary/quarantine tanks, but risks increase with tank size and sensitivity of livestock.
When is tap water “good enough”? For small, fish-only tanks or temporary setups, dechlorinated tap water may be sufficient—especially if your municipal water is low in contaminants. For reef tanks or long-term success, RODI water is strongly recommended.
Common failure modes from poor source water:
- Persistent nuisance algae (phosphates, silicates)
- Coral stress or poor growth (heavy metals, nitrates)
- Unexplained livestock losses
How to check your tap water quality: Use a TDS (Total Dissolved Solids) meter to get a quick read on water purity, or review your municipal water report for contaminants. If you notice recurring algae or livestock issues, your water source may be the culprit.
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.
Aeration and mixing ensure all salts are fully dissolved, stabilize pH, and allow excess CO2 to off-gas. Incomplete mixing can leave undissolved salt, cause cloudy water, or result in unstable water chemistry.
How long should you mix and aerate? Most salt mixes recommend at least 1–2 hours of mixing and aeration, but some reef keepers prefer to mix for 12–24 hours for maximum stability—especially for large water changes or sensitive tanks.
Signs of incomplete mixing: Cloudy water, visible residue, or undissolved crystals at the bottom of the bucket.
Temperature matters: Use room temperature water. Very hot or very cold water can affect how salts dissolve and may alter water chemistry.
What if you skip this step? Skipping proper mixing and aeration can cause pH swings, precipitation of calcium/carbonate, and stress to your tank’s inhabitants.
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.
Use a hydrometer to measure the specific gravity
A hydrometer is an inexpensive tool that floats in the water to measure specific gravity. It’s easy to use but can become inaccurate over time due to salt buildup or bubbles. A refractometer is a handheld optical device that measures salinity more precisely and is less prone to error, though it costs more.
Temperature effects: Specific gravity readings are temperature-dependent. Some hydrometers are calibrated for a specific temperature (usually 77°F/25°C). If your water is much warmer or cooler, readings may be off. Many refractometers are temperature-compensated.
Maintenance tips: Rinse your hydrometer or refractometer with fresh water after each use to prevent salt buildup. Periodically calibrate refractometers using calibration fluid or distilled water. Cross-check your device with a known standard or at your local fish store occasionally to ensure accuracy.
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.
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.
While not required to make saltwater, keeping a log of your specific gravity readings and water changes can help you spot gradual drifts, equipment failures, or the impact of new practices. Look for trends such as consistent drops or rises in salinity, or sudden changes that may indicate a problem. Good record-keeping can make troubleshooting much easier down the road.
Troubleshooting
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.
| Problem | Likely Cause | Solution |
|---|---|---|
| Cloudy water after mixing | Incomplete mixing, undissolved salt, or impurities in source water | Mix and aerate longer; ensure water is at room temperature; check for residue and stir well |
| Specific gravity too low | Not enough salt mix added, or inaccurate measurement | Add small amounts of salt, dissolve thoroughly, and retest |
| Specific gravity too high | Too much salt mix added | Add purified water in small increments, mix, and retest |
| Residue at bottom of mixing container | Incomplete dissolution, low-quality salt mix, or cold water | Mix longer, use room temperature water, or try a different salt mix |
| Livestock stress after water change | Large or abrupt change in salinity, temperature, or pH | Match temperature and salinity closely to tank; make gradual changes |
Conclusions
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:
- How to set up a saltwater aquarium (a step-by-step guide)
- The 9 most important reef tank water parameters
- How to acclimate your fish
- Selecting a reef tank salt mix
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.