Aquarium Salt Calculator

What aquarium salt is and when to use it

Aquarium salt is plain sodium chloride sold for freshwater tanks, and the DailyMed listing for API aquarium salt lists the active ingredient as sodium chloride. The dose is measured by weight against the water volume, and the math is the same regardless of brand: a 0.1 percent solution in freshwater is about 1 tablespoon per 5 US gallons. The Merck Veterinary Manual: providing a home for fish page goes over the broader topic of adding salt for salinity control in saltwater tanks, which uses a different scale (about 30 to 33 grams per liter).

This calculator covers three freshwater dose levels:

• General tonic (0.1 percent). 1 tablespoon per 5 gallons. Used when adding new fish, during transport, or as a routine low dose in fish-only tanks.
• Treatment (0.3 percent). About 1 tablespoon per 1.7 gallons. Used for nitrite spikes, ich and minor skin flukes.
• Strong treatment (0.5 percent). About 1 tablespoon per gallon. For short-term dips or a strong ich outbreak; not for a planted community tank.

Salt is a tool, not a routine. The general tonic is enough for most situations once the biofilter is mature. Higher doses should match a specific problem and end when the problem clears.

Worked examples

Worked example: a 20 gallon tank at the general tonic dose

20 US gallons is 75.7 liters. Salt at 0.1 percent by weight in 75.7 L of water is:

• Grams: 75.7 x 1000 g/L x 0.001 = 75.7 g
• Tablespoons: 75.7 / 18 = 4.2 tbsp
• Teaspoons: 4.2 x 3 = 12.5 tsp
• Cups: 4.2 / 16 = 0.26 cup

That matches the 1 tablespoon per 5 gallons rule of thumb, and it is the dose API and most other brands print on the box.

Worked example: a 55 gallon tank at the treatment dose

55 gallons is 208 liters. Salt at 0.3 percent is 208 x 0.003 x 1000 = 624 g. In kitchen units:

• Grams: 624 g
• Tablespoons: 624 / 18 = 34.7 tbsp
• Cups: 34.7 / 16 = 2.2 cups

Dissolve it in a gallon of tank water in a clean bucket before adding it back to the tank, and add it over a few hours so the fish adjust.

The math behind the calculator

How the math works

Percent in this context means grams of salt per 100 grams of water. A US gallon of water weighs about 3,785 grams. So a dose of P percent in G gallons of water is:

grams of salt = G x 3785 x P / 100

For practical use, 1 tablespoon of aquarium salt weighs about 18 grams, 1 teaspoon is 6 grams, and 1 cup is 16 tablespoons or 288 grams (the salt is denser than water, so a cup of dry salt weighs more than a cup of water). The calculator rounds to the nearest tenth of a tablespoon so the dose is easy to measure with a kitchen spoon.

Which fish and plants tolerate salt

Which fish and plants can take salt

Salt is a working tool, not a routine. For a planted community tank the goal is usually no salt and a mature biofilter that handles ammonia and nitrite. For a fish-only livebearer or African cichlid tank, salt is a useful part of regular care.

When to dose and when to skip

When to use salt, and when to skip it

• New tank cycling. A small dose of 0.1 percent salt reduces the toxicity of nitrite while the bacteria colony establishes. It is not a substitute for a proper cycle, but it helps the fish survive the first 4 to 6 weeks.
• Nitrite spike in an established tank. Salt lowers the amount of nitrite the fish absorb across the gills, which can keep them alive while you fix the cause. Dose at 0.1 percent and find the source of the spike.
• Ich (white spot). Salt at 0.3 percent, slowly raised, kills the free-swimming ich stage without harming most community fish. Combine with a few degrees of temperature rise if your fish tolerate it.
• External flukes or skin parasites. Short salt dips (1 to 5 minutes at 0.5 to 1.0 percent in a separate container) can clear mild cases.
• Routine prevention. Skip it. Most healthy, established tanks do not need salt. Use it as a treatment, not a habit.

Common mistakes when dosing salt

Common mistakes when dosing aquarium salt

• Dumping dry salt into the tank. Undissolved salt can burn a fish on contact, and a pile on the substrate traps ammonia. Always dissolve the dose in a cup of tank water first.
• Adding salt to a planted tank. Vallisneria, Amazon swords, java fern and most stem plants yellow and melt at 0.1 percent or higher. For a planted tank, use extra water changes and a mature biofilter instead.
• Salting scaleless fish. Corydoras, otocinclus, and most loaches react badly to salt. The safe limit for them is closer to 0.05 percent, which is below any meaningful dose. Skip the salt for these species.
• Forgetting to redose after a water change. Salt leaves with the old water. A 25 percent change removes 25 percent of the salt in the tank, so add 25 percent of the original dose to the new water.
• Using iodized table salt. Iodine and anti-caking agents in table salt irritate fish gills over time. Use aquarium salt, which is plain sodium chloride per the DailyMed listing for the common brands.
• Raising the salt too fast. Sudden changes in salt stress fish. Add the dose over a few hours for a routine tonic, and over a day or two for a treatment dose.
• Leaving salt in the tank after treatment. Most treatment courses end in 7 to 14 days. Long-term dosing in a community tank can stress plants and scaleless fish, and it builds up if you forget the redose step.
• Measuring by spoonful, not by weight. A kitchen tablespoon of fine salt is about 18 g, but coarse aquarium salt can run 20 to 22 g per tablespoon. When in doubt, weigh the dose on a kitchen scale, especially for a 0.5 percent treatment where 1 g off is a 5 percent swing.

The calculator above uses 18 g per tablespoon, the working value for a level tablespoon of standard aquarium salt. If you are weighing on a scale, use the grams column directly.