Understanding Nitrite

This article was first written for and published by Koi Magazine in 2009

Nitrite – what is it and how does it get into a pond?

The subject of nitrite and its effects in Koi ponds can become very technical in explanation but it is very important that as a Koi keeper you understand the implications of its presence as if the nitrite level in your pond is too high it can be detrimental to the health of your Koi.  For this reason, I will try and explain it in a way that everybody can understand and stick to basics.  After the Koi in your pond have digested their food they excrete the waste into the water and the most harmful portion of this is released as ammonia.  The ammonia passes into the filter where bacteria grow and these bacteria (nitrosomas) use the ammonia as a food source.  The nitrosomas bacteria then excrete nitrite as a by product and at high levels nitrite is toxic to your Koi.  Fortunately, another group of bacteria in your filter (nitrobacter) eat the nitrite and they convert it into nitrate, which is a lot less toxic.  There is also yet another group of bacteria which break down the nitrate into free ammonia, or nitrogen gas, which is released into the atmosphere when the surface of the water comes into contact with air.  Therefore, the more you feed your fish, the more ammonia is released – the more ammonia  that is released, the more the nitrosomas bacteria have to eat and so the faster they grow and reproduce and the more nitrite is released.

How does nitrite build up?

Nitrite levels can build up if, for example, a lot of ammonia is suddenly being produced, perhaps due to overfeeding or the introduction of a large number of new fish to a pond, which are then being fed. The colony of bacteria in the filter that have developed to cope with the existing levels of ammonia and nitrite, suddenly can’t cope with the increased levels and so the levels of ammonia and nitrite rise and become toxic to your Koi.   During the cold winter months, the bacteria in the filter die back and when spring comes along and the water warms up, it can take a few weeks for the bacteria colony to be completely effective – this is the time to slowly build up the amounts that you feed your Koi and not go mad by throwing too much food into the pond because your Koi suddenly become ravenous.

What are the effects of high nitrite levels?

Nitrite levels over 0.15 milligrams per litre can be damaging to Koi and high levels of nitrite can result in something called “brown blood disease” which prevents the transport of oxygen into the blood and results in the fish suffocating despite apparently high levels of oxygen in the water.  Signs that the fish are suffering from the effects of high nitrite levels or nitrite poisoning are that they might rub themselves (flash) on the bottom of the pond or lay with fins clamped to their sides on the floor of the pond – but still come up for food and afterwards return to the pond floor.  When nitrite poisoning is advanced the fish will be gasping for air at the surface and eventually die.  In an emergency, nitrite toxicity can be reduced by adding salt to the pond (which puts chloride ions into the water) and maintaining a level of salt concentration of 0.3% in the water will usually alleviate the effects of moderate to high levels of nitrite.  Also, stop feeding for a few days and carry out a partial water change before adding salt.

How can high nitrite levels be prevented?

Sudden rises in nitrite levels can be prevented by feeding a consistently even volume of food on a regular basis, so that you don’t get sudden fluctuations in ammonia production.  Good filter maintenance and avoiding chemical and environmental shocks can also prevent rises.  Regular water changes of 10% per week and good filter and pond maintenance are not just good for the health of your Koi but are also good for the health of your filter.  Whatever filter media you use make sure that you clean it weekly and if you have to wash the media, use water from your pond and not from your tap as chlorine in your tap water will kill the bacteria in your filter.  Chemical shocks can be the result of a medication that has been used to treat your fish and chemicals such as Potassium Permanganate or Formalin will not just kill parasites but will also kill off some of the bacteria in the filter.   What I would recommend is that if you do have to shock your filter system with a chemical to treat a parasitic infection in your pond, then it is important to slow down your feeding rate and then increase it gradually over a period of a week or so, so that you filter can build up a healthy population of bacteria again.

How can a pond be managed to provide healthy levels of nitrite?

Until a filtration system is completely and fully mature, the ammonia and nitrite levels will peak and trough  and these fluctuations can take place at different times throughout the course of a day, so just because you have a slightly high nitrite reading at one particular point in the day, it doesn’t mean that you necessarily have a problem and it just could be that the bacteria that eat the nitrite (nitrobacter) haven’t yet multiplied into a population that can digest all the nitrite that is being released.  It will only take a short time for that population to react to the increase in nitrite however and with careful feeding you can build up a healthy population within a few weeks without any detrimental effect on your Koi. The rise in the population of bacteria in a filter is totally temperature and environment dependant and oxygen plays a major part too because the bacteria in your filter (nitrosomas and nitrobacter) are oxygen breathing.   Biological filtration is an oxygen using process and a sufficient amount should be provided not just for your Koi but also for the bacteria in your filter.

What can you do if nitrite levels become too high?

It should never be your aim to get your pond water to have zero ammonia, nitrite or nitrate because the bacteria in your filtration system would never have anything to eat and would starve – if this happened your filter would stop working and since if you feed your fish, as you must, they will inevitably excrete ammonia, it is an unrealistic aim.  If however you feel the levels of nitrite are getting too high and are not showing any signs of reducing, or are at levels that you feel are toxic to your fish, then the immediate addition of salt will help reduce the toxicity of the nitrite.  Salt works because it increases the levels of chloride ions in the water and at concentrations of 0.3% there are enough chloride ions in the water to block the nitrite molecules and stop them from diffusing into the Koi – it won’t stop all of them though so this can only be a temporary measure.

We use, and recommend that you use, PDV (Pure Vacuum Dried) salt but you mustn’t dump the salt into the pond, it must be dissolved before it goes in.  We add a large amount of salt to a bowl, fill the bowl with pond water, give it a good stir, let the solution settle and pour off the dissolved salt solution into the pond water.  (I have in the past, when I worked for a time in Germany, known salt tablets to be used and these were suspended at the side of the pond in a net bag so that the fish had no way of coming into direct contact with high salt concentrations.)  Table salt contains iodine and is toxic to fish, as are salts with arsenic based anti caking agents, so you must be very careful when choosing which salt to use.  (Your local Koi shop should stock the correct type of salt.)

Salt, like any chemical, must only be used as a last resort and I would highly recommend that when you do use it, you use  a salt test meter to check that the dosage is correct and is between  1 to 6 kilos of salt per cubic metre (or 220 gallons) of water.  If you haven’t got an electronic salt meter, there are alternative tests available from marine aquarium shops or you can even use a hydrometer.  The meter should read between 0.1% and 0.6%.  Koi can survive higher concentrations of salt for a reasonable length of time but I wouldn’t recommend levels above this for inexperienced Koi keepers.

Adding salt has an almost immediate effect on the nitrite levels.  If you have inadvertently added too much salt to the pond, then you must immediately try and rectify that problem and bring the level down to below 0.7% as fast as you can by doing water changes.  Or in drastic situations, take the fish out and put them into fresh water.

You can add commercial cultures of nitrobacter to your filter to help boost the population of nitrite eating bacteria, and reduce nitrite levels more quickly, but these are expensive and some are useless as nitrobacter are oxygen breathing and they won’t be effective unless they are supplied in an oxygenated environment. The other way to reduce nitrite is to add Zeolite – a mineral which absorbs ammonia and as a consequence leads to a reduction in the amount of nitrite. However, you should not use Zeolite and salt at the same time because salt will have the effect of releasing the ammonia stored in the Zeolite.

Salt must only be used, like any chemical, as a last resort.  And levels must be monitored and controlled to provide the desired concentrations.  As soon as any problem has been alleviated the salt concentrations should be brought back to normal as soon as possible with water changes.

What else is salt good for?

  1. At concentrations up to 0.3% or 3 kilos to the cubic metre, it can be used to provide a stress relieving environment for fish during or after transportation.
  2. It can also trigger an increase in mucus production therefore boosting the fish’s external (slime coat) immune response.
  3. It can prevent secondary infections from bacteria and fungus.
  4. At higher concentrations, from 0.5 – 0.6 %, salt can have anti parasite properties and is particularly effective against costia, trichodina, white spot and even flukes* (See update note below).
  5. These parasites have yet to develop a way of dealing with high salt levels and it is the release of sodium ions, from the sodium chloride part of the chemical, which has an effect on the sodium pump cells in the parasites bodies (the sodium pump controls the influx of water in most aquatic animals) causing an increased uptake of sodium ions and osmotic water loss which leads to the collapse of the parasites cells and the death of the parasite.
  6. If a Koi is suffering with ulcers, the damaged tissue will allow water to be absorbed into the body much more quickly which creates stress on the fish’s internal organs and can lead to dropsy.  Adding salt to the water reduces this process by making the salt content in the water closer to the fish’s own salt content which means the fish’s body doesn’t have to work so hard and it is less stressed.

*Update 2019 – recommended dose rates for salt:

0.75% (7.5ppt) Tosai introduction, Chilodonella and White Spot (Ichthyophthirius) treatment

0.9%(9ppt) Costia (Ichthyobodo) treatment (for 3 days).

We no longer recommend treatment of Trichodina with salt as some strains are now salt resistant.

Instructions for adding salt

  1. Salt, like any chemical, must only be used as a last resort – Only use it as a medication.
  2. Use PDV (Pure Vacuum Dried) – table salt contains iodine and is toxic to fish, as are salts with arsenic based anti caking agents.
  3. Don’t just pour salt directly into the pond – it must be dissolved before it goes in.
  4. Put your salt into a bowl and fill it with pond water, give it a good stir, let the solution settle and pour off the dissolved salt solution into the pond water.
  5. use a salt test meter to check that the dosage is correct and is between  1 to 6 kilos of salt per cubic metre (or 220 gallons) of water. If you don’t have a salt meter, calculate pond volume in metres cubed at 1kg per metre cubed to 0.1% salt concentration. Therefore, 30kg of salt is added to 10 metres cubed (2,200 gallons) for 0.3%.
  6. Koi can survive higher concentrations of salt for a reasonable length of time but I wouldn’t recommend levels above 0.6% for inexperienced Koi keepers.
  7. Don’t use any other medication with salt other than those recommended by the manufacturer and particularly, don’t use it at the same time as Formalin (formaldehyde) as this can lead to the death of all the fish in the pond.