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What nitrite does to a fish

Hi Everyone,

     I'm building a large pond and while reading up on pond maintenance I
came across this piece of information on nitrite:

Brown blood disease occurs in fish when water contains high nitrite
concentrations. Nitrite enters the bloodstream through the gills and turns
the blood to a chocolate-brown color. Hemoglobin, which transports oxygen in
the blood, combines with nitrite to form methemoglobin, which is incapable
of oxygen transport. Brown blood cannot carry sufficient
amounts of oxygen, and affected fish can suffocate despite adequate oxygen
concentration in
the water. This accounts for the gasping behavior often observed in fish
with brown blood disease, even when oxygen levels are relatively high.

Sodium chloride (common salt, NaCl) is used to "treat" brown blood disease.
Calcium chloride
can also be used but is typically more expensive. The chloride portion of
salt competes with nitrite for absorption through the gills. 

As a general rule, catfish producers strive to maintain at least 100 ppm
chloride in pond waters as "insurance" against high spikes of nitrite