Adding Aquarium Salt to a Freshwater Tank to Get Ride of Nitrites

Aquarium salt is commonly used to treat sick and injured freshwater fish. Salt encourages the healing of wounds and kills off external parasites. It is also used to prevent nitrites from entering a fish's body via the process of osmosis. Freshwater fish can tolerate a low level of salt, but not as much as a salt water-dwelling fish would. Unlike sea water, which contains a high concentration of dissolved salt ions, fresh water has a low to little concentration of the substance.

  1. About Salt

    • Sodium chloride is sold as kosher salt, sea salt and table salt. Marine salt is the type of salt used on roads in the winter to melt ice. Aquarium salt is table salt that has no anti-clumping agents or iodine added to it. Unlike chlorine, salt or sodium chloride does not naturally evaporate or leave the water. This means that if you keep adding salt without replacing the evaporated water or changing 25 percent of the water in your freshwater tank every week, its levels (measured in parts per million or ppm) will continue to rise and eventually get to such high levels that it is toxic to your fish.

    Freshwater Salt Treatments

    • Salt is added to freshwater tanks to treat parasites, such as Ich, and to prevent disease. Salt is also used to lower a fish's ability to uptake nitrites, which are the byproduct of the natural break down of fish waste by beneficial bacteria. The ability of sea salt or aquarium grade salt to cure many fish diseases, and prevent stress caused by a transfer to a new fish tank, leads many to believe that it is the ultimate cure-all of freshwater fishes. While it does perform what seem to be miracles for the health of a fish, too much salt in tank water can actually harm them.

    Fish Osmosis

    • Osmosis is the movement of water molecules through a selective permeable membrane of a cell in a fish's body. Water molecules move through cell membranes to an area that has a higher concentration of solutes, such as sodium, chloride and nitrites in an attempt to maintain osmotic pressure. Osmotic pressure is the water pressure needed to have an equal amount of solutes on either side of the cell's membrane. A fish's body contains sodium and chloride ions that are transported by the blood. Due to osmotic pressure, the tank water will attempt to regulate the difference in osmotic pressure between the fish's body and the water, so that the sodium and chloride ions in the fish match the levels of salt in the tank water. This is why aquarium salt does not get rid of nitrites, but prevents a fish's body from absorbing them via osmosis.

    Nitrite Uptake

    • By using the principle of osmosis, you can add salt to tank water to prevent nitrite poisoning in your fish. Nitrite is measured in parts per million or ppm. It is toxic when it reaches a level of 0.1 ppm in the tank water. As long as there is 30 times more chloride ions than nitrite ions in the water, the fish's body will not absorb the nitrite. If the nitrite is 0.1 ppm, you would need to add enough aquarium salt to raise the chloride levels to 3 ppm. Measure the salt levels with a water testing kit, and adjust as necessary by adding 1/2 tsp. aquarium salt. Wait overnight for the salt to completely dissolve and disperse in the water before testing again. Continue adding 1/2 tsp. aquarium salt until the chloride levels reach 3 ppm.