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.

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 10-20% 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?

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.

Adding salt to the pond prevents the nitrite molecules from fixing to the red blood cells in the Koi, allowing more oxygen through, thus counteracting the effects of high nitrite. If your Koi are experiencing the effects of high nitrite in your pond, we would recommend you raise the salt level in the pond to 0.75%. This level is held for 1 week, before being slowly reduced by your regularly weekly water changes.

You can read more about salt, how to safely use it, and the benefits in our article:  https://cuttlebrookkoifarm.co.uk/pages/when-and-how-to-use-salt