Don’t “a-salt” your fish! Know when and how to use salt safely when treating freshwater fishes.

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Some more specific information on anti-caking agents and iodine in salt below, researched by one of my students, (now) Dr Cyrus So:

Table salt contains an anti-caking agent, sodium hexocyanoferrate, which prevents the salt from absorbing too much moisture from the air, which would otherwise cause the salt to form hard lumps. One paper states that ferrocyanide is “harmful to aquatic life with long lasting effects”. In a study, the LC50 of ferrocyanide for fish is 20mg/L after 4 days. LC50 stands for the toxic concentration required to kill 50% of a given population. Given that table salt contains 100mg/kg ferrocyanide, and the normal recommended amount of salt to add to water is 5g/L, this equates to a ferrocyanide concentration of 0.5mg/L, which is much lower than the LC50. But we note that the LC50 was for fish subjected to the 20mg/L concentration for 4 days. There is no published information on the effects of 0.5mg/L on fish from possible accumulation or chronic exposure. For this reason, I would not recommend using salt that contains anti-caking agents.

Iodised salt contains iodine which can be in the form of iodide (I^–) or iodate (IO₃^–), at 20-40mg/kg of NaCl. In fish, excess iodine can induce colloid goitre (disease of thyroid gland due to excess iodine). The LC50 for rainbow trout for IO₃⁻ is 220 mg/L, and for I⁻ is 860 mg/L. If we use salt at 5g/L, it would equate to 0.2mg/L. So this means it should be pretty safe to use. Moreover, if the iodised salt contains iodine in the form of iodide, this would be even safer because it is not as stable. An opened packet of such salt would rapidly lose its iodine content through the process of oxidation and sublimation. This said, my recommendation is to still avoid using iodised salt if possible. Because again, the toxic levels are given as LC50, and for 4 days. So smaller amounts, and with chronic exposure, we do not know its cumulative effect.