Nitrite is formed in the second stage of the Nitrogen Cycle. It isn't as toxic as Ammonia, but long term exposure can cause problems. Nitrite is also called brown blood disease, because a Koi's blood will turn brown from a increase of methemoglobin. Methemoglobin causes the red blood cells to have a reduced Oxygen carrying capacity. Adding salt (Sodium Chloride) to your pond will combat Nitrite poisoning. 99% pure rock salt works great. Elevated levels of Nitrite can be brought down with additional biological filtration and water changes. Nitrite is converted into Nitrate by the bacteria, Nitrobacter.
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