HOW TO MANAGE NITRITE POISONING
Fortunately, there is an effective way to blunt the harmful effects of elevated nitrite that doesn’t involve changing lots of water- you add salt (sodium chloride) to the water. The
chloride in the salt acts to” block” the ability of nitrite to enter though the gills of the fish and thus to cause the harm inside the fish it might. So it is possible to manage elevated nitrite over the short term using salt in relatively small amounts.
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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. Maintaining at least a 10 to 1 ratio of chloride to nitrite in a pond effectively prevents nitrite from entering catfish.” 1
It should be noted that the Merck Veterinary manual suggests a lower ratio of chloride:
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In freshwater outdoor ponds, the concentration of Cl– can be increased (by the addition of salt) to a ratio of 6 parts Cl– to 1 part NO2. This will dramatically decrease the percentage of Hgb converted to methemoglobin, resulting in immediate relief to the fish and stopping most further mortality within 24 hr.”
2
Since the amount of salt needed to produce either 6 or 10 times the chloride as nitrite is minimal, this author prefers to use the higher ratio of 10 to 1 in order to be more certain of obtaining the needed relief. One should also be aware that studies indicate that, whether one changes water or uses chloride to counter the effects of nitrite toxicity, it will still take between 24-72 hours for nitrite already inside fish to be completely eliminated. Preventing further nitrite from entering usually solves the problem.
WHAT SALT TO USE 
Plain old table salt is just fine for use here. Do not worry if it says
Iodized or if it says it contains Anti-caking agents. The amount of either of these in the salt is so minimal one would pickle their fish long before these ingredients would be doing any harm. Read here for facts about table salt and fish:
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PERFORMING DILUTED NITRITE TESTING
The problem with dealing with elevated nitrite is that the typical aquarium kits do not go high enough to let us know how much nitrite may really be in any tank. And this makes fish in cycling more complex and more work than fishless. You will likely need to know how to do diluted nitrite tests. For this you need a way to create an accurate mix of your tank water and some amount of
pure water, i.e. distilled or reverse osmosis/deionized (ro/di) water. You should be able to find a gallon of distilled water in the supermarket. Some fish stores sell ro/di water.
You will need a clean measuring cup as well. You will use this to mix different solutions of tank and
pure water to be able to test for nitrite. You
do not want to use your tap water for dilution purposes as it will often contain things that can cause test result be inaccurate.
The reason for using a measuring cup is that it is important to get the proportions of the mix as close to dead on as possible. The advantage of measuring using a cup from which you will only use a few ml is that the potential for mixing errors is way less of an issue in 8 ounces than in 5 or 10 ml of water. What you will do is start by making a 50/50 mix (4 ounces each) of tank and
pure water. Then you pour 5 ml of this into the little test tube and then do the nitrite test. Multiply the result of the test by 2 to get the actual ppm in your tank.
However if this 50/50 diluted test result is still at the kit’s maximum level and the test kit reads to a maximum level of 8 ppm or less, you will have to do another dilution. The easiest way is to start with ¼ cup of tank water and ¾ cup of pure water. Test this mix and multiply the result by 4 this time. Alternatively you can use ½ cup of the initial 50/50 mix and then add ½ cup of pure water to this. The result will also be ¼ tank and ¾ pure water. If this test is also at the maximum number, don't worry about a further test, just do a huge water change (at least 50%) ASAP. After doing the water change, test for nitrite and then add the required amount of salt to deal with that amount of nitrite.
You will need to continue testing for nitrite because any further rise may mean that either water changes are needed and/or that further salt additions are needed.
CALCULATING HOW MUCH SALT TO ADD
PPM is a measure of concentration in water. You cannot weigh ppms. However, 1 mg/l is almost the exact equivalent in water to 1 ppm. So one can use ppm and mg/l interchangeably in this case. You can weigh milligrams.
To add 10 mg/l of chloride for every ppm of nitrite in the water, use the following steps:
1. Multiply your nitrite test reading by 10. This will give you the needed mg/l of chloride you need to add.
2. Calculate the actual volume
in litres of the water in your tank. If your volume is in gallons you must convert this into liters. (As a rule, using the advertised volume of the tank at about 85% will put you in the right ballpark.) 1 gallon = 3.875 litres
3. Multiply the number in #1 above by the number of liters of water in #2 above to get the total number of mg of chloride you will need to add.
4. Because salt is roughly 2/3 chloride, you must multiply the number calculated in #3 by 1.5. You now know how many mg of salt you should add to the water. Dividing this number by 1,000 will convert this amount to grams which are easier to weigh for most people.
5. Do not add the dry salt directly to the tank. Remove some tank water to a container and mix the salt in that, then add the salt water to the tank spreading it around the surface.
Hint: We have calculated a handy conversion from grams to volume so one can measure in tea or table spoons which most folks are likely to have while a gram scale is not. The following calculations were made using an Ohaus triple beam scale: ¼ teaspoon of salt shaker sized table salt weighs 2 grams.
The readings used in this article are for API and similar type test kits which measure in total ions. Some kits will measure using a different scale, they are only measuring the nitrogen ions. You can tell when a kit reads just the nitrogen ions by the way they state things. The typical kits will say they measure Total Ammonia (NH3 + NH4), Nitrite (NO2) or Nitrate (N03). Kits that measure only the nitrogen ions will usually say they measure Total Ammonia-Nitrogen (NH3-N + NH4-N), Nitrite-Nitrogen (NO2-N) or Nitrate-Nitrogen (NO3-N).
Just like one can convert distances between miles and kilometers, one can convert between the total ion scale and the nitrogen ion scale. Since this article is using the total ion scale, if you have a kit that reads in –Nitrogen (-N), you must multiply your results as follows:
