How Much Salt per Gallon for a Saltwater Aquarium?

Plan on roughly half a cup of aquarium salt mix per US gallon of RO/DI water as a starting point, then add more until a refractometer reads your target. For a reef tank that target is 35 ppt, a specific gravity of about 1.026, and most mixes need a bit more than half a cup to get there. The cup is only the first step. The salinity reading decides how much salt you actually use.

Why is half a cup per gallon only a starting point? #

Instant Ocean, one of the most common mixes, gives exactly this direction on its product page: half a cup per US gallon for small batches. The same page says 1.5 pounds makes 5 gallons at a specific gravity of 1.022, and lists a recommended range of 1.020 to 1.024 at 77°F. That range suits a fish-only tank. A reef runs higher, close to natural seawater, which NOAA puts at an average of about 35 parts per thousand.

So if a brand’s label dose lands at 1.022 (about 29 ppt), you’ll need roughly a fifth more salt to reach 1.026 (about 35 ppt). Hobbyists report the same thing from the other direction: half a cup per gallon of many mixes ends up somewhere between 1.021 and 1.023.

Cups are also imprecise. Grain size differs by brand, a scooped cup holds more than a poured one, and salt that has absorbed humidity packs differently. Weighing is more repeatable if you mix large batches often.

How much salt do you need for a water change? #

Here’s a starting table at half a cup per gallon. Mix this in, let it dissolve, then measure and add small amounts until you hit your target.

The “likely extra” column assumes your mix behaves like the label example above, reaching 1.022 at the label dose. Some mixes are denser and need less. After a couple of batches, write down the total you really used per gallon and start from that number next time.

To size the batch, multiply your system’s water volume by the percentage you’re changing. A 10% change on a 75 gallon display is 7.5 gallons, and if you run a sump, count its water too. Our guide on how often to change aquarium water covers picking the percentage.

How do you mix saltwater step by step? #

1. Start with RO/DI water in a clean, food-grade container that has never held soap or chemicals. Tap water brings phosphate, silicate and chloramine, and an RO/DI unit is only as good as its cartridges, as we cover in how often to replace RO/DI filters.
2. Add a heater and a small pump or powerhead, and bring the water to tank temperature.
3. Add the salt gradually into moving water. Dumping it all in at once can leave a hard clump on the bottom.
4. Let it mix. Many hobbyists run the pump for several hours to a day. Check your salt’s label, since some brands say theirs is ready sooner.
5. Measure salinity at tank temperature once the water is clear and the salt fully dissolved.
6. Adjust. Add salt in small amounts to raise salinity, or RO/DI water to lower it. Let it dissolve and measure again.
7. Check the temperature one more time, then do the water change.

Why does my salinity keep changing after mixing? #

Usually because the salt hasn’t fully dissolved, the temperature is still moving, or the refractometer isn’t calibrated. Measure only after the water has cleared and the temperature has settled. Calibrate your refractometer with 35 ppt calibration fluid. RO water only sets the zero point and doesn’t check the part of the scale you actually use. Our guide to what salinity a reef tank should be has the ppt to specific gravity conversions.

Can you mix salt directly in the tank? #

Only in a brand-new tank with nothing living in it. Freshly added salt swings pH and salinity quickly, and grains sitting on sand or rock can burn anything they touch. Once there’s livestock in the tank, always mix in a separate container and add finished water.

Does the salt brand matter? #

For fish-only tanks, less than people argue about. For reefs it matters more, because each brand mixes to its own alkalinity, calcium and magnesium levels. A brand that mixes to 11 dKH alkalinity will push a tank running at 8 dKH upward with every water change. Test one fresh batch so you know what yours makes, and stick with the same brand so water changes don’t swing your numbers.

How do you keep water changes consistent? #

The point of measuring every batch is that each water change puts the same water back in. Salinity that wanders from batch to batch, 1.024 one week and 1.027 the next, stresses corals and invertebrates more than a steady number slightly off target.

Fish Tank Tracker keeps a water change log, by percentage or by volume, next to your salinity readings in ppt, which it charts against a 34 to 36 ppt band. You can note which salt you used and what the batch measured, and set a weekly or custom reminder for the next change. It works offline and needs no account.

Frequently asked questions #

Can I use table salt or grocery store sea salt? #

No. Aquarium salt mix contains magnesium, calcium, potassium, carbonate and trace elements in roughly the proportions of seawater. Table salt is nearly pure sodium chloride, often with iodide and anti-caking agents added, and it won’t support fish or corals.

How much salt per liter for saltwater? #

Half a cup per US gallon works out to about 2 tablespoons per liter as a starting point. As with gallons, finish by measuring with a refractometer and adjusting to your target.

How long does mixed saltwater last? #

Mixed saltwater kept covered, circulated and at room temperature is commonly stored for a week or two. Some reefers see alkalinity drop in batches that sit much longer, so test anything that’s been waiting for weeks before you use it.

How do I lower salinity if I added too much salt? #

In a mixing container, add RO/DI water in small amounts, mix and remeasure. If the high salinity is in the tank itself, correct it gradually with extra fresh water over several days rather than one big change, since sudden salinity swings stress livestock.