Aquarium Salt: Measuring Doses and Removing Salt
By Richard James · Updated 27 October 2026
Content revised: 27 October 2026.
Salt dosing depends on the intended purpose, concentration, exposure time and the species being treated. A larger dose is not automatically a better treatment. This guide explains the arithmetic and how water changes affect salt already in an aquarium.
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Choose a supported concentration first
University of Florida aquaculture guidance describes several uses of sodium chloride, but stresses that concentration, exposure duration and species tolerance matter. Plants and some fish may not tolerate the proposed salinity. A preset name such as “support” or “severe infection” does not establish that the dose is appropriate for your aquarium.
Our salt calculator retains its existing tablespoon presets. They calculate quantities; they are not a diagnosis or a species-specific treatment recommendation. Follow an applicable product label or advice from a fish-health professional before selecting a treatment concentration. Do not use a universal half-dose rule for sensitive species.
Mass gives a clearer concentration than spoonfuls
For a concentration specified in grams per litre:
For example, 40 litres × 1 gram per litre = 40 grams. This is an arithmetic example, not a recommended treatment. Spoonfuls depend on crystal size and how the spoon is filled, so do not assume a universal grams-per-tablespoon conversion.
Evaporation and replacement water
When water evaporates, the dissolved salt remains and its concentration rises. Topping up to the original water level with salt-free water restores the previous concentration; it does not require replacing salt. Removing well-mixed aquarium water removes a corresponding share of the dissolved salt.
If maintaining an established treatment concentration, replacement salt is based on the water removed, not a fresh full-tank dose. If removing the treatment, use salt-free replacement water and account for salt that remains.
Why three 25% water changes do not remove all the salt
Assume the tank is well mixed, each change removes 25% of the water, the tank is refilled to the same volume with salt-free water, and no salt is added between changes:
| Water changes | Original salt remaining |
|---|---|
| Before any changes | 100% |
| One 25% change | 75% |
| Two 25% changes | 56.25% |
| Three 25% changes | 42.19% |
| Four 25% changes | 31.64% |
Remaining concentration = starting concentration × (1 − fraction changed)number of changes. For unequal changes, multiply the remaining fraction for each change instead.
Starting at 1 g/L, three 25% changes leave approximately 0.422 g/L. This corrects our earlier statement that three such changes return salinity to zero. Choose a suitable removal schedule for the livestock and water conditions; this table is not an instruction to make rapid changes.
What has changed in this guide
We have removed the universal salt-dip recipe, broad cure claims and unsupported “lazy kidneys” explanation. A dip is a different procedure from a prolonged bath and should not be improvised from a general calculator preset.
Sources and further reading
- University of Florida IFAS: The Use of Salt in Aquaculture
- Dose My Tank: calculation method and limitations
External guidance checked 27 October 2026. The dilution examples above follow the stated mass-balance assumptions.
Check volume and units
Use measured water volume when available and check the calculator’s decor setting before calculating a quantity.
Open Salt CalculatorWritten by Richard James
Aquarist, author, and creator of ShrimpKeeper.co.uk. Helping hobbyists achieve professional results through precision dosing.
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