Nitrites kill a LOT of fish every year and there's a video at Youtube

Nitrite: Second Fish Killer from Nitrogen Cycle Control Diagnosis Symptoms

Nitrites – A Tenth the Level of Ammonia Kills

Written by Dr Johnson
Nitrites: Second Waste product in the Cycle, Low Numbers KILL! –

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. (Test kit)
2) Partial water changes 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 is only temporary, only lasts maybe 3-6 weeks. Well, that’s okay because you’re supposed to have cultivated a “Cycle” in that time for sure.

Video on the Cycle and how to wrap it up overnight. 

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.

Ammonia is converted into Nitrite by Nitrosomonas

The nitrogen cycle
Beneficial bacteria reduce toxic nitrogen compounds in fish waste

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 BACK 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 a Buffer. 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 under sizing 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.