Care Guides · Jul 12, 2026 · By Fish Tank Volume Calculator Team
How Much Aquarium Salt Should I Add?
How much aquarium salt to add per litre for tonic and treatment doses, which fish tolerate salt, and how to remove it safely, with a free salt calculator.
Aquarium salt is one of the most misunderstood products in the hobby. Used correctly, it can ease stress and help treat certain ailments in freshwater fish. Used carelessly, or in a tank full of salt-sensitive fish and plants, it does more harm than good. The key to using it well is dosing by your actual water volume, not by guesswork or a scoop.
This guide gives you the standard doses, explains the difference between a tonic and a treatment dose, and covers the fish and plants that should never see salt. It also shows how to remove salt once you are done.
The quick answer
Aquarium salt, also called tonic salt, is dosed by water volume. A level tablespoon weighs roughly 15 grams.
- Mild tonic dose: 1 tablespoon per 19 litres (5 US gallons)
- Medium dose: 1 tablespoon per 11 litres
- Strong treatment dose: 1 tablespoon per 8 litres
For your exact tank, the aquarium salt calculator turns your volume and chosen dose into grams and tablespoons. Because dosing depends on the real water in the tank, work out your true volume first with the water volume calculator, since substrate and decor mean the tank holds less than its rated size.
Tonic versus treatment doses
The right amount depends on why you are using salt.
| Use | Dose | Notes |
|---|---|---|
| General tonic / stress | 1 tbsp per 19 L | Gentle, occasional support |
| Mild ailment support | 1 tbsp per 11 L | Short course, monitor fish |
| Stronger treatment | 1 tbsp per 8 L | Short term only, salt-tolerant fish |
Always start at the lower end and only increase if needed. Dissolve the salt in a cup of tank water before pouring it in, so it does not settle on the substrate or contact fish as a concentrated grain.
Which fish and plants tolerate salt
This is the part people skip, and it is the one that matters most. Many fish and plants are sensitive to salt.
- Sensitive fish: many scaleless species such as certain catfish (corydoras, some plecos) and loaches react badly to salt.
- Sensitive plants: most live aquarium plants dislike salt and can melt at treatment doses.
- Tolerant fish: livebearers like mollies and guppies handle salt well, and it is sometimes used deliberately for them.
If your tank is planted or houses scaleless fish, treat salt as a last resort and consider a hospital tank for the affected fish instead. Never dose a community tank blindly.
Salt does not evaporate
A crucial detail: salt only leaves the tank through water changes, never through evaporation. When water evaporates, the salt stays behind and the remaining water becomes saltier. So top up evaporation with plain dechlorinated fresh water, never more salt water, or the concentration creeps up over time.
This also means you should track how much salt is in the tank. If you dose and then do a partial water change, you remove some salt and may need to re-dose the fresh water you added to keep the concentration steady during a treatment.
How to remove aquarium salt
To take salt back out, do repeated partial water changes with fresh, unsalted water. Each change dilutes what remains. Two or three changes of 25 to 30 percent over a few days will bring a treated tank back to near zero. Base the change amount on your real water volume so you know how much salt each one removes.
Frequently asked questions
How much aquarium salt per gallon? A mild tonic dose is about 1 tablespoon (15 grams) per 5 US gallons, which is roughly 19 litres. Use the salt calculator for stronger doses and exact amounts.
How much is a tablespoon of aquarium salt in grams? A level tablespoon of aquarium salt weighs roughly 15 grams, though it varies slightly with grain size.
Do all fish tolerate aquarium salt? No. Many scaleless fish like corydoras and loaches, and most live plants, are salt-sensitive. Check your species before dosing a whole tank.
Does aquarium salt evaporate out of the tank? No. Salt stays behind when water evaporates, so top up with plain fresh water and only remove salt through water changes.
Should I dose salt to tank size or water volume? To the real water volume, after subtracting substrate and decor, so you do not overdose a tank that holds less water than its rated size. Find it with the water volume calculator.
The bottom line
Aquarium salt is a useful tool when dosed carefully and a risk when it is not. Confirm your real water volume with the fish tank volume calculator and water volume calculator, pick a dose for the job, and let the aquarium salt calculator turn it into an exact amount. Start low, watch your fish, and remember that salt only leaves the way it came in, through water changes.