Nitrites - A Tenth the Level of Ammonia Kills
Written by Dr Johnson

Nitrites: Second Waste product in the Cycle, Low Numbers KILL! - By Dr. Erik Johnson
Nitrite is an intermediate metabolite in the CYCLE. Nitrite binds fish Red Blood Cells causing gasping and "brown blood disease". Extension agents without testing gear can make a presumptive diagnosis of Nitrite poisoning by simply cutting a catfishes' head off, and the blood will be brown.

Fish that die with their gills widely flared, died of Nitrite induced "met-hemoglobin-emia"
Ways to combat nitrite intoxications are as follows:
1) Know the problem exists, a simple test will tell.
2) Partial waterchanges will lower Nitrite levels
3) Addition of salt (non iodized 1 teaspoon per gallon all at once) will inhibit uptake of Nitrites. (Preferred)
Interestingly, the protective function of salt lasts maybe 3-6 weeks. Well, that's okay because you're supposed to have cultivated a "Cycle" in that time for sure.

4) Methylene Blue at bottle doses may also help, but not entirely reverse the Met-Hemoglobin-emia.
In many instances, the filter needs cleaning or upgrading.
Feeding should be suspended or reduced.

If you are running a drip irrigation water replacement system, you will not need to worry about Nitrites.

Article Number Two on Nitrites

Nitrites - Brown Blood Disease
Written by Dr Erik Johnson

Ammonia is converted into Nitrite by Nitrosomonas
So, when you set up a new pond, and the fish produce Ammonia; that ammonia is reduced to Nitrite, the subject of this article.
Nitrite is converted into NitrAte (plant food) by Nitrobacter.
Nitrites cause reddening of the fins and irritation of the gills, gasping + excess mucus. A simple test kit can detect Nitrite.
Nitrites also bind the fish Red Blood Cells resulting in suffocation and "Brown Blood Disease".
-> Nitrite toxicity is temporarily reduced by the addition of salt at one teaspoon per gallon of water.
Nitrites can be created from Nitrate under anaerobic conditions. (Deep sand, glutted filters, stalled sand filters etc.)
Nitrites can be controlled with wet-dry filtration, constant replacement of a little water all the time to the tune of ten percent per week, and traditional water changes as needed.
Nitrite toxicity is weakly reversed by addition of Methylene Blue.
+ "The reversal of Nitrite poisoning by salt is not permanent. Work at Auburn University showed the protection varied among fish specis and could last up to eight weeks." ~ Doc Johnson

+ Note From Doc Johnson
"When I have Ammonia or Nitrite problems, one of the first things I do is raise the Total Alkalinity of the pool with baking Soda or AquaClarus. Then I wll suspend feeding and increase aeration until the numbers come down." ~ Doc Johnson

+ Nitrite accumulations with certain kinds of filters can be due to common undersizing mistakes. It may also be that you're not giving sufficient oxygen or Calcium to the beneficial bacteria living in your filter. Before you get too frustrated with your filter, make sure you get some calcium carbonate for it.