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Adding Salt to a Freshwater Aquarium

When and When not to Add Salt

50 gallon aquarium

Published - 20061213, Revised:

The practice of adding salt (a.k.a. sodium chloride, rock salt, table salt, solar salt, aquarium salt) to freshwater aquariums has been around almost as long as the hobby. There are several reasons why hobbyists add salt to the aquarium, stress reduction, medicating, adding chloride, and for brackish water fish.

It has become a common practice for employees of big box stores to tell all of their freshwater customers to add a teaspoon of salt per 10 gallons (38 l). This is not a practice most advanced hobbyists partake in, nor one recommended. Before you add salt to a freshwater aquarium, you should understand why you are doing so and any possible side effects.


Most wild populations of freshwater fish and plants in the hobby come from rivers and lakes with very little detectable salt. Freshwater fish are adapted to water with a salt content that is measured in parts per million (ppm) versus seawater, measured in parts per thousand (ppt).

My local water supply comes from surface water, and it averages less than 3 ppm of total sodium. That means that out of one million parts, less than 3 are salt. In many cases, freshwater has very low sodium content, and in some areas of the world, like tropical rainforests, it can be so minute that it is undetectable.

Fish and Plants

Freshwater fish and plants have evolved to live in an environment with very little salt; because of this, some are very sensitive to salt. Most freshwater plants do not tolerate much salt, and if you are trying to keep live plants, salt, as a general rule, should be avoided. Some species of fish from very soft water, like Plecostomus (suckermouth catfish), do not tolerate much salt either, and the addition of salt to the aquarium with these fish should be avoided.

Salt as a Medicine

The most important use of salt is for medicating freshwater fish for some types of ectoparasites (parasites on the outside of the fish).

Some of the common parasites that are known to be killed by salt are ich (Ichthyophthirius), costia, and anchor worms (Lernaea). To kill these parasites with salt, you need to add a lot of salt, about 1½ cups of salt per 10 gallons (38 l) to the aquarium, not just a teaspoon per 10 gallons. The "Specific Gravity" measured with a standard marine aquarium hydrometer should be between 1.005 and 1.009 or 7 to 13 ppt. If treating for anchor worms closer to 1.009 would be best. This level of salt needs to be maintained for at least 3 weeks. Not all freshwater fish can tolerate this much salt. Generally, Central American cichlids and livebearers, African rift lake cichlids, and koi can tolerate this much salt.

Saltwater dips are also common for eradicating some external parasites. A saltwater dip with a specific gravity of 1.009 to 1.023 for 30 minutes to 2 hours kills Gyrodactylus (skin flukes). Whenever treating freshwater fish with salt, you should monitor the fish closely for excessive stress and return the fish to freshwater when necessary. In many cases, treating external parasites with other types of medication may be safer for the species of fish you are trying to cure.

A therapeutic amount (1 teaspoon per 10 gallons) of salt is sometimes added to the quarantine/hospital tank for fish that have damaged skin or have lost a lot of scales. This small amount of salt will aid the fish with osmoregulation by helping the fish maintain the salt level in its blood while the fish's wound heals. Once the wound has healed, I recommended removing all the salt from the aquarium by doing a nearly 100% water change. Salt does not evaporate, so you must do a water change to remove it.

Zeolite and Salt

Zeolite is often used in aquariums to remove ammonium/ammonia from the solution. When salt is added to the aquarium, it prevents the mineral from removing the ammonium/ammonia. Therefore zeolite is only effective in freshwater. Zeolite can be recharged by soaking it in saltwater. When soaked in saltwater, it exchanges the ammonium/ammonia with salt.

As a general rule, salt should not be added to a freshwater aquarium. Salt is an effective medication, provided the fish you are treating is salt tolerant. Salt should never be used in aquariums with live plants. Alternative medications should be used with fish that are not salt tolerant.