How the aquarium salt calculation works
For sodium chloride in fresh or brackish water, the amount to add is the change in concentration multiplied by the water volume. With concentrations in grams per litre, the answer comes out in grams.
Example: 100 L of water, a current concentration of 0 g/L and a target of 1 g/L gives (1 − 0) × 100 = 100 g of salt. That is a calculation example, not a recommendation to dose any aquarium at 1 g/L: safe levels depend on the species, plants and invertebrates and on why the salt is being added.
Use the actual water volume. Substrate, rocks, decorations and equipment displace water, so a tank sold as 100 L may hold noticeably less. The calculator can subtract a percentage if you only know the tank’s capacity.
Freshwater aquarium salt
Aquarium salt (sodium chloride) is used as a short-term aid when fish are stressed or exposed to nitrite. Chloride competes with nitrite at the gills, and a salinity of 0.1–0.3% blocks nitrite uptake (WSAVA/VIN). API recommends 1 rounded tablespoon for every 5 US gallons (20 L) when setting up, changing water or treating disease.
Salt does not evaporate and is not filtered out (API). Top up evaporation with plain water and only add salt for the new water at each change, or the level keeps rising.
| Water | 1 g/L (0.1%) | 3 g/L (0.3%) | API label rate |
|---|---|---|---|
| 20 L (5.3 US gal) | 20 g | 60 g | 1.1 rounded tbsp |
| 40 L (10.6 US gal) | 40 g | 120 g | 2.1 rounded tbsp |
| 60 L (15.9 US gal) | 60 g | 180 g | 3.2 rounded tbsp |
| 100 L (26.4 US gal) | 100 g | 300 g | 5.3 rounded tbsp |
| 200 L (52.8 US gal) | 200 g | 600 g | 10.6 rounded tbsp |
Not every tank tolerates salt
Aquarium salt, pond salt and marine salt mix are not interchangeable
- Freshwater aquarium salt is mostly sodium chloride. API’s is made from evaporated sea water and is meant for set-up, water changes and disease recovery, not as a permanent additive.
- Sodium chloride (NaCl) works for freshwater treatments if it is plain salt, without iodine, anti-caking agents or other additives.
- Pond salt is sodium chloride sold in bulk for ponds; check the bag for additives.
- Marine salt mix recreates sea water. Dissolved to SG 1.026, Instant Ocean provides about 400 mg/L calcium and 1,320 mg/L magnesium as well as sodium chloride. Never use it as a freshwater salt treatment, and never use sodium chloride to make sea water.
- Reef salt mix is a marine mix enriched to support corals and other invertebrates, such as Instant Ocean’s Reef Crystals.
- Brackish set-ups usually use a marine salt mix at reduced strength, so the fish also get the minerals that pure sodium chloride lacks.
Mixing marine and reef saltwater
For marine and reef tanks, follow your salt brand’s mixing chart and measure the result; do not apply the freshwater formula blindly. Salt mixes contain moisture and different ion blends, so the weight for a given salinity differs between brands. Red Sea’s chart works out at about 1.08 g per litre for each ppt; Instant Ocean’s label (1.5 lb in 5 US gallons for SG 1.022) works out nearer 1.12 g. The calculator uses 1.1 g per litre per ppt unless you enter your product’s own dose.
Always add salt to the water, mix with a pump until it is fully dissolved, bring it to tank temperature and measure. Never mix salt in a tank that holds animals (Instant Ocean, Red Sea). Adding about 90% first and topping up after measuring avoids overshooting.
| Aquarium type | Salinity | Dose per litre | Dose per 5 US gal |
|---|---|---|---|
| Fish only | 31.0 ppt | 33.4 g | 1.4 lb |
| Non-coral invertebrates | 33.5 ppt | 36.0 g | 1.5 lb |
| Corals | 35.5 ppt | 38.2 g | 1.6 lb |
Salt concentration units: g/L, mg/L, ppt, ppm, % and SG
Grams per litre and milligrams per litre measure salt per volume of water. Parts per thousand (ppt) and parts per million (ppm) measure salt by weight: 10,000 ppm means 1% of the water’s weight is dissolved salt (USGS). In dilute fresh water, 1 g/L is almost exactly 1 ppt; in sea water the denser water makes 35 ppt equal to about 35.8 g/L.
USGS classes water with under 1,000 ppm of dissolved salts as fresh, 1,000–3,000 ppm as slightly saline, 3,000–10,000 ppm as moderately saline and 10,000–35,000 ppm as highly saline. Ocean water has about 35,000 ppm.
| Level | g/L | ppt | ppm | % | SG at 25 °C |
|---|---|---|---|---|---|
| 0.1% freshwater treatment | 1 | 1.0 | 1,000 | 0.1% | – |
| 0.3% freshwater treatment | 3 | 3.0 | 3,000 | 0.3% | – |
| Natural sea water | 35.8 | 35 | 35,000 | 3.5% | 1.0264 |
Specific gravity depends on temperature and the instrument
Specific gravity compares the density of your water with pure water, and both change with temperature. Holmes-Farley gives natural sea water (35 ppt) as SG 1.0264 on the 77 °F (25 °C) standard used by refractometers, but 1.0269 on the 60 °F standard of many glass hydrometers. A 60 °F hydrometer read at 86 °F that shows 1.023 is really about 1.027.
Red Sea’s own chart shows the same effect: one mix reads 1.021 at 24 °C but 1.025 at 7 °C. The calculator converts SG with the international equation of state for seawater (EOS-80), using your instrument and water temperature.
| Salinity | 30 ppt | 32 ppt | 33 ppt | 34 ppt | 35 ppt | 36 ppt |
|---|---|---|---|---|---|---|
| SG at 25 °C | 1.0226 | 1.0241 | 1.0249 | 1.0256 | 1.0264 | 1.0271 |
Salt for water changes, refills and evaporation
Salt leaves the tank only with the water you remove; it does not evaporate and is not filtered out (API). Keep two jobs apart. Replacing what you removed means dosing only the new water, at the tank’s concentration. Adjusting the whole tank to a new target means working out the concentration left after the change, then dosing all the water.
Example: a 100 L tank at 1 g/L has a 25% water change. Dosing the 25 L of new water at 1 g/L takes 25 g and keeps the tank at 1 g/L. Refilled with plain water instead, it falls to 0.75 g/L; three such changes leave about 0.42 g/L.
When water evaporates, the salt stays behind and the concentration rises. Top up evaporation with plain dechlorinated water (RO/DI water for marine tanks). Adding salt with top-up water makes the level climb with every refill.
Lowering salinity: a 200 L marine system at 37 ppt that should be 35 needs 200 × (1 − 35 ÷ 37) = 10.8 L swapped for fresh RO/DI water. Split large corrections over several days.
Weighing salt versus using spoons and cups
Weigh salt in grams or kilograms for accurate results. Spoon and cup measures depend on the product and grain size: USDA lists fine table salt at 6 g per teaspoon, 18 g per tablespoon and 292 g per cup, while coarse aquarium and pond salt weighs less per spoon. Instant Ocean’s ½ cup per gallon works out at about 136 g, or roughly 17 g per tablespoon.
API’s label uses rounded tablespoons, which hold more than a level one. If you use spoons, weigh one level spoonful of your salt and enter it in the calculator.
