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[JohnO]

Here's something to chew over :-) It was a reply to a question about the Nitrogen cycle.

Note the reference to Nitrite and its effect on marine fish as against freshwater fish.


" The nitrogen cycle is essentially the same in freshwater and marine situations and I have described it here: The Nitrogen Cycle. About the only thing that varies in the cycle is the genera and species of bacteria that perform the mineralisation, nitrification and denitrification. In both freshwater and marine conditions, nitrite is produced and oxidised as part of the cycle.

The main difference between freshwater and marine conditions is the toxicity of the nitrite. Many fish in freshwater cannot tolerate even small concentrations of nitrite. The recommended level for salmonids is less 1.6 mg/L of nitrite. Other fish have a lower tolerance and some freshwater fish can tolerate over 100 mg/L of nitrite. This all changes in saltwater due to the high concentration of chloride which inhibits the uptake of nitrite across the gills. In fact, the recommended treatment for high nitrite in freshwater aquaria and aquaculture ponds is the addition of salt, which adds chloride.

The 24 hour LC50 for spotted sea trout in water with a salinity of 14 ppt is over 3,000 mg/L of nitrite. (LC50 refers to the lethal concentration which would result in the death of 50% of the individuals exposed to that concentration, so 24 hour LC50 means that if the fish were exposed to 3,000 mg/L of nitrite for 24 hours, 50% would die.) The "safe level" of nitrite is less than the 24 hour LC50, but even if 1% is assumed, the safe level would be around 30 mg/L.

The more commonly used indicator is the 96 hour LC50 which is generally around 10 times the safe level. The 96 hour LC50 for European sea bass is around 300 mg/L, so a safe level of 30 mg/L could be assumed. In other studies, fish have shown no acute toxicity to nitrite at as high as 5,000 mg/L. "