Aquarium salt mix calculator

How the aquarium salt mix calculator works

Salinity is often measured as specific gravity, where clean fresh water reads 1.000 and full strength seawater near 35 parts per thousand reads about 1.026. The calculator converts your target specific gravity to parts per thousand, then works out the salt needed from the fact that roughly half a cup of marine mix, about 130 to 140 grams, brings one US gallon up to a reef target. It scales that to your volume and target and shows the result in pounds, grams and approximate cups.

Brands vary a little, so treat the figure as a close starting point rather than an exact dose. Always confirm the final reading with a refractometer or a good hydrometer, since guessing salinity is one of the most common causes of stress in marine and brackish tanks. Mix in a clean bucket or a dedicated mixing station, not in the display tank with livestock present.

As an example, a 20 gallon batch aimed at 1.025 comes out near five and a half pounds of salt, or about ten cups, at a target of roughly 33 parts per thousand. Scale that up or down and the amount moves with both the volume and the target, which is why a small change in specific gravity changes the salt noticeably on a big batch.

Picking a target salinity

Reef tanks with corals usually sit at 1.025 to 1.026, close to natural seawater, because corals and many invertebrates need full strength salinity. Fish only saltwater tanks are often kept a little lower, around 1.020 to 1.023, which is easier on the fish and uses less salt. Brackish tanks for species like mollies, figure eight puffers or archerfish need only a fraction of that, roughly 1.005 to 1.010, so enter a lower target for those.

Pick one target and hold it. It is the stability, more than the exact number within these ranges, that keeps livestock healthy. Decide based on what you keep, set the calculator to that specific gravity, and aim to hit the same reading every time you mix a batch.

Specific gravity, salinity and ppt

You will see salinity written two ways. Parts per thousand is the actual amount of salt, so natural seawater is about 35 ppt. Specific gravity is how dense the salt water is compared with pure water, which reads about 1.026 for that same seawater at reef temperature. The two move together, so you can use either, but be consistent about which one you follow.

Temperature affects the reading, which is why a good refractometer is temperature compensated and why you calibrate it with a known solution. A cheap swing arm hydrometer works in a pinch but drifts, so treat its number as a guide and confirm important readings with a refractometer.

How to mix saltwater step by step

Start with reverse osmosis or RODI water in a clean, dedicated container, never a bucket that has held soap or chemicals. Add a heater and a powerhead so the water is at tank temperature and moving. Pour the salt into the water gradually, not the water onto the salt, and let it circulate until it is completely clear, which can take an hour or more.

Once it looks clear, test the salinity with your refractometer and adjust: add a little more salt to raise it, or a little more fresh water to lower it. Only use the batch when the salinity and temperature match the tank. For a reef, letting freshly mixed saltwater circulate overnight before use lets the chemistry settle.

Salt mix by tank size

These are rough amounts of marine salt for a full batch at a reef target near 1.025. A lower target for a fish only or brackish tank uses less. Confirm with the calculator and a refractometer for your exact target.

• 10 gallon: about 3 pounds
• 20 gallon: about 5 to 6 pounds
• 40 gallon: about 11 to 12 pounds
• 55 gallon: about 15 pounds
• 75 gallon: about 21 pounds
• Brackish at 1.007: roughly a quarter of these amounts

Why use RO water

Tap water carries chlorine, metals, phosphate and nitrate that feed algae and upset reef chemistry, so most saltwater keepers mix with reverse osmosis or RODI water instead. Starting from pure water means the only things in your saltwater are the salt mix and what you add on purpose, which makes salinity and parameters far easier to control.

If you cannot get RO water, dechlorinate tap water before mixing and be aware that it may already contain nitrate or phosphate. For a fish only or brackish tank this is usually workable, but for a reef, RO or RODI water is worth the effort.

Keeping salinity stable

Marine fish, and especially corals and invertebrates, dislike sudden salinity swings. Change only part of the water at a time, match the new saltwater to the tank in both salinity and temperature, and mix each batch to the same target. The calculator helps you hit that same figure every time so your changes do not nudge the tank up or down.

One key point catches many beginners: only fresh water evaporates, leaving the salt behind, so the tank slowly gets saltier as it evaporates. Top up evaporation with fresh RO water, not saltwater, and add saltwater only when you remove some in a water change.

Common salt mixing mistakes

The frequent mistakes are mixing salt directly in the display tank, guessing salinity instead of measuring it, and adding cold or freshly mixed water straight to livestock. Each one shocks fish and invertebrates. Pouring water onto the salt rather than salt into the water can also cause clumping and cloudy batches that take longer to clear.

Topping up evaporation with saltwater is another common error that slowly drives salinity up over weeks. Measure every batch with a refractometer, mix in a separate container, and top up only with fresh water, and most salinity problems disappear.