Formalin and potassium permanganate are frequently recommended to treat parasites, and both are valuable. The concurrent use of salt with either chemical can be very damaging, if not lethal. Goldfish are particularly prone to die if subjected to the combined use of Formalin and salt, although koi are less susceptible. Prudence suggests that salt be used either before, or after, a treatment with another chemical such as Formalin, potassium permanganate, or an anti-parasitic drug. For years, both hobby and professional fish keepers have used very low salt water concentrations when moving their fish from one pond to another, or back and forth from a show.
Salt is perhaps the single most important chemical for reducing transport stress in fish.
Even extremely low amounts, such as 1.5 lbs for every 100 gallons, are beneficial to fish during period of transport stress. Low levels of salt concentration are also used by fish keepers who maintain their fresh water fish in tanks or ponds where no biological balance is attempted. The low concentration of salt will help keep the fish healthy, warding off disease and infection, and keeping their slime coat in top condition. If the pond is equipped with biological filtration and some plant life, then there is no reason to keep a constant, low-level of salt concentration in the water. In fact, many plants will not tolerate salt in the water.