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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 hardness, and for fish commonly found in brackish water. 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 hobbyist 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.
Freshwater Most wild populations of freshwater fish and plants in the hobby come from rivers and lakes that have very little if any detectable salt. Freshwater fish are adapted to water with salt content that is measured in parts per million (ppm), versus seawater that is measured in parts per thousand (ppt). My local water supply comes from surface water and it averages less than 3 ppm total sodium. That means that out of one million parts, less than 3 are salt. In many cases freshwater has very low salt content, and in some areas of the world, like tropical rain forests, it can be so minute that it is undetectable.
Fish and Plants Freshwater fish and plants have evolved to live in an environment that has very little salt, and because of this some are very sensitive to salt. Most freshwater plants do not tolerate much salt at all, 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 (Sucker Mouth Catfish) do not tolerate much salt either, and the addition of salt 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 know to be killed by salt are ich (Ichthyophthirius), Costia, Anchorworms (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 Anchorworms 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 is know to kill Gyrodactylus (skin flukes). When ever 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 shin or lost a lot of scales. This small amount of salt will help the fish with osmoregulation by helping the fish maintain the salt level in it’s blood while the fish’s wound heals. Once the wound has healed, I recommended removing all the salt from the aquarium by doing a near 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 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 are salt tolerant. Salt should never be used in aquariums with live plants. Alterative medications should be used with fish that are not salt tolerant.