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Treatment Rules

The main factor in successful koi-keeping lies in the maintenance of good water quality. Ammonia, nitrite and pH levels in the pond should be checked regularly. However, what is seldom mentioned is the details of how 'poor water chemistry' can affect the health of our fish, with the most common answer being 'it causes stress'. Through understanding how a specific substance present in the water can affect the fish's physiology, we can recognise certain behaviour the fish show as a response. With an understanding of the parameters which control water quality (zero ammonia and nitrite level through biological filtration, and pH maintenance via water changing), this article intends to reveal how some of the biological processes the fish relies on are affected by certain chemicals naturally present in ponds.

Ammonia – Maximum concentration in pond = ZERO.

Ammonia (also seen as NH3 and NH4+) is produced by fish and excreted as waste through the gills. It is also one of the most common areas of water quality to degrade. Depending on the pH and temperature of the water, ammonia molecules can exist in two forms: at a low pH and temperature, ammonia is in an ionised form (NH4+, an ammonium ion) as it binds with an extra hydrogen atom. Despite ammonia being highly toxic to fish, this ionised form of ammonia is safe for fish. However, as the temperature and pH increase, the ammonium ion begins to lose hold of the extra hydrogen atom, and eventually this is lost creating unionised or “free” ammonia. Ammonia in this unionised form is the highly toxic form, even at small quantities, as the molecule will then bind to another atom to become ionised again.

Fish exposed to a concentration of unionised ammonia in their environment will suffer from ammonia poisoning. An influx of unionised ammonia molecules will bond with other ions, including salt, which causes the fish to effectively become more permeable to water and disrupting its osmotic balance. Similarly, unionised ammonia molecules can bind to haemoglobin (the oxygen carrying molecule in a fish’s blood) in the fish’s red blood cells and prevents oxygen from binding to these same sites. If this occurs, the fish will have a reduced ability to carry oxygenated blood around their body and will begin to be unable to perform basic functions such as movement. If severe enough, this can cause rapid and excessive growth of new cells at the gills (known as gill hyperplasia) as the body tries to compensate for the lack of oxygen, but this actually further impedes oxygen uptake. Ammonia molecules also cause the destruction of mucus membranes (leaving the fish susceptible to parasitic infection, and can cause degradation of the brain and central nervous system through their primary and secondary effects. The effects of ammonia on fish are seemingly endless, but are never positive!

Fish suffering from ammonia poisoning can show a wide variety of symptoms, including flicking and rubbing on objects, gasping at the surface (as fish try to take in more oxygen), clamped fins and may isolate themselves at the bottom of the pond. You may also see blood streaked fins and eroded body structures including fins.

If you see any of these symptoms you should test the water quality of the pond immediately. If there is a measurable ammonia concentration (anything above 0 parts per million (ppm)), perform a substantial water change (at least 50%) and repeat until the concentration of ammonia falls to 0 ppm. Any level of measurable ammonia can indicate a problem with the biological filter, so you should also increase their numbers in the pond by using a bacterial treatment, such as Pond Mature or Koi Care Filter Bugs. Of course, regularly checking water quality with a test kit (such as our Pondlab - 200 Multi-test Kit) can alert you to a potential problem before your fish begin to show symptoms of their suffering, and is the most important thing must do for your fish.

Nitrite – Maximum concentration in pond = ZERO.

Nitrite (also seen as NO2) is produced by the breakdown of ammonia by nitrifying bacteria (eventually becoming nitrate). It is extremely toxic to fish due to the effect it has on haemoglobin. If fish are exposed to a concentration of nitrite in their environment, these nitrite molecules will bind to the haemoglobin in the fish’s blood, converting it to methaemoglobin. Methaemoglobin has a reduced ability to carry oxygen (unless the nitrite binds to a haemoglobin molecule which is already carrying oxygen, in which case it becomes excellent at picking up oxygen but then will not release it for use by the fish), reducing the amount of oxygenated blood available to the fish for daily, basic functions. This process is more commonly known as methaemoglobinaemia.

Methaemoglobinaemia has some obvious symptoms. As the concentration of nitrite in the pond increases, the proportion of haemoglobin converted to methaemoglobin also increases and the fish will begin to effectively suffocate. In an attempt to take on more oxygen to counteract this, fish will gasp at the surface to take on more oxygen. Additionally, whereas haemoglobin is bright red in colour, the conversion to methaemoglobin turns the colour to a dull brown, and can most easily be seen at the gills.

As with ammonia poisoning, if you see either of these symptoms you should check your water quality immediately. If a nitrite concentration above 0 ppm is recorded, perform repeated, significant water changes until the concentration falls to 0 ppm. Any level of measurable nitrite can indicate a problem with the biological filter, so you should also increase their numbers in the pond by using a bacterial treatment, such as Pond Mature or Koi Care Filter Bugs. Of course, regularly checking water quality with a test kit (such as our Pondlab - 200 Multi-test Kit) can alert you to a potential problem before your fish begin to show symptoms of their suffering, and is the most important thing must do for your fish.

pH - Ideal value of pond = Between 7.0 and 8.0.

pH is the measure of alkalinity or acidity of a liquid, and ranges from 0 (highly acidic) to 14 (extremely alkaline) with 7.0 being neutral. In a general pond community, the pH of the water should be between 7 and 8, but this will depend on your species specifically. Temporary, minor fluctuations outside this preferred range has little harmful effect to fish, but prolonged exposure to a pH that is either too high or too low can lead to significant health problems.

If the acidity of a pond increases and the pH falls below 6, fish will begin to suffer from acidosis. The acidity of the water causes intense irritation to the fish’s skin and they will begin to produce excess mucus in response. The acidic water can also cause the erosion of vulnerable, delicate tissues such as the gills, and this leaves the fish susceptible to parasitic, bacterial and fungal infection. Additionally, erosion of the gills will reduce oxygen uptake and will lead to hypoxia, causing the fish to effectively suffocate. This is worsened because the acidic conditions also reduce the efficiency of haemoglobin at carrying oxygenated blood around the body. Without getting too bogged down in the science behind it, the acidity alters the structure of the haemoglobin making it unable to carry as many oxygen molecules, exacerbating the oxygen supply issues caused by erosion of the gills.

In all ponds, there will be varying amounts of metal ions, including copper, aluminium and zinc. Typically, at a neutral pH, these metals are insoluble and cause no harm to your fish. If the pH drops, these metal ions will start to dissolve in the acidic water, and have high toxicity to fish when in solution. The lower the pH, the more quickly these ions will dissolve, and you will quickly start to lose fish.

At the other end of the scale, a high pH means your water is very alkaline. Fish left in an alkaline environment for extended periods will begin to suffer from alkalosis. The reactions to this are very similar to acidosis: the alkalinity will begin to erode the skin and gills, leaving the fish at greater risk of bacterial, fungal or parasitic infection, and so fish will produce more mucus from their skin and gills. This reduces oxygen uptake, and begins to suffocate the fish. As was described earlier, ammonia is in its unionised form (NH3) at higher pHs, and will reduce the number of oxygen binding sites available on the haemoglobin molecules in the fish’s blood as these are taken by unionised ammonia molecules, suffocating the fish further.

The symptoms for acidosis and alkalosis are very similar. If the pH changes rapidly and significantly, fish will become highly agitated and thrash and throw themselves around the pond, potentially even leaping out of the pond (causing further harm, but that’s another story!). Due to the lack of oxygenated blood in their body, fish will gasp at the surface in an attempt to take in more oxygen, and may rub on various objects in an attempt to relieve the irritation caused to their skin and gills. Gradual changes in pH will not have such obvious behavioural symptoms, with fish appearing more lethargic and listless, and will have a steady increase in their mucus excretions. Due to their increased susceptibility, fish are likely to succumb to numerous parasitic and bacterial infections, even if you haven’t added any new fish or made any changes recently.

As with nitrite and ammonia, if you see any of these symptoms you should check your water quality immediately. If the pH is incorrect, perform a partial water change daily until the value is correct. Unfortunately, all significant changes in pH will cause stress to fish, even those that are helpful and making it safer, so corrective pH changes must be carried out gradually. You can also add a pH adjustment product, such as our Koi Care - KH Buffer Up, which will boost the carbonate hardness of the water (see below), and will re-stabilise the pH. Of course, regularly checking water quality with a test kit (such as our Pondlab - 200 Multi-test Kit) can alert you to a potential problem before your fish begin to show symptoms of their suffering, and is the most important thing must do for your fish.

Hardness (GH and KH) - Ideal values in pond = GH: 10°dH and above, KH: 6°dH and above.

Water hardness is determined by the amount of certain dissolved minerals in the water. If you live in a ‘hard’ water area, your water will have a higher concentration of dissolved minerals than if you live in a ‘soft’ water area. For fish keepers of all types, there are two measures of hardness which should be checked regularly: general hardness (GH) and carbonate hardness (KH).

General hardness is the measure of all the dissolved minerals in the water and represents the total hardness of the water (determining whether your water is hard or soft). Minerals are, of course, vital for fish health and growth, so the general hardness of the pond should be above 10°dH to ensure all your pond inhabitants have sufficient minerals for their everyday lives. You can supplement the GH level of your pond by adding a mineral mix, such as our Koi Care - GH Minerals Up.

Carbonate hardness measures the proportion of minerals in the pond, which contribute to the water’s natural pH buffer system (alkalinity) to resist acidification of the water. Fish consistently release ammonia into the water which is then broken down into nitrate. These nitrates tend to form acids, and, when coupled with the influx of free hydrogen ions which are removed during the nitrification of the ammonia, decrease the pH of the pond. Additionally, fish will release carbon dioxide as they respire, which forms carbonic acid as it dissolves, causing further lowering of the pH. Over time, this pH drop is completely normal, and if it weren’t for the probability of fish suffering from acidosis and becoming very ill, this acidification would not be a problem.

Fortunately, bodies of water such as ponds have this KH buffer in place. This is mainly comprised of carbonates, bicarbonates and hydroxides, and can ‘absorb’ the acids and neutralise them, maintaining the pH. These minerals are not limitless, and will eventually be used up by the constantly produced acids, leaving your pond at risk of a pH crash (where the pH plummets to a highly acidic degree, and can kill your pond inhabitants from acute acidosis). It is therefore essential to maintain a high KH level, as higher levels of KH will be able to neutralise more acids and prevent pH fluctuations and crashes for longer. If you live in a very hard water area, this is something you may not need to worry about too much, as the buffering minerals will be replaced during water changes from your tap water. Conversely, if you live in an area with soft water, the buffering minerals in your tap water are unlikely to be high enough to maintain the KH through water changes alone, leaving the pond very susceptible to pH fluctuations and a pH crash. Regardless of your water type, you should test the KH regularly to prevent any foreseeable accidents.

To maintain an appropriate KH level, it is important to perform small water changes, and add a mineral supplement if necessary (such as our Koi Care - KH Buffer Up). Although it is vitally important to not let your KH get too low, it is also important that this is not too high, or the water can become very alkaline and fish may begin to suffer from alkalosis (although this is much less likely than problems caused by a low KH). A KH value between 6°dKH and 16°dKH is ideal, but may be slightly higher depending on your tap water hardness. The most important thing is to ensure this level does not drop, and is why you should test your water quality regularly using a test kit (such as our Pondlab - 200 Multi-test) to ensure the GH and KH are consistent and you aren’t putting your fish at risk.

Chlorine – Maximum concentration in pond = ZERO.

Chlorine is put into tap water as a disinfectant to keep our drinking water clean and safe. However, it does not distinguish between the microorganisms that could harm us in our tap water and the fish we bought for our ponds. The effect of chlorine is two-fold. Firstly, it will kill off many of the nitrifying bacteria in the filter, causing problems with water quality including ammonia increases. Secondly, chlorine burns and erodes delicate tissues on the fish, including their gills and reduces oxygen uptake. Eventually, the gills will become so eroded that the fish will be unable to take in any more oxygen and it will die. Fish in this scenario will begin to shake nervously, lose colour rapidly and may even attempt to jump out the pond to get away, before becoming listless and weak. Additionally, the reduction in the population of filter bacteria will cause ammonia and nitrite levels to rise, resulting in further stress and degradation of your fish.

As well as chlorine, tap water also contains heavy metals and chloramines. Chloramines will behave in a similar way to chlorine, but with the added problem of the release of ammonia as the molecules split. Heavy metals are toxic to fish, will accumulate over time in sensitive areas such as the gills and internal organs, eventually becoming so toxic that the fish are killed.

To prevent any fish loss or discomfort due to unsafe tap water, always use a dechlorinator when filling or refilling your pond. The chemicals contained within these dechlorinators, such as our Pond - Tap Water Chlorine Remover or Koi Care - Chlor-Go, reduce and neutralise chlorines and chloramines to prevent them from corroding tissues, and will bind to heavy metal molecules to prevent them being taken up by fish. Dechlorinators must be added prior and during each water change, and are vital in protecting your fish from unnecessary stress.


Poor water quality (i.e. A measurable level of ammonia, nitrite or chlorine, low KH and unstable pH) is more often than not the main cause of ill health in fish. The old saying “look after the water and the fish will look after themselves” is very true, although is often shrugged off by many hobbyists. However, the symptoms of fish in a pond with poor water quality are very similar to those of fish suffering from a parasitic or bacterial infection: gasping at the surface, flashing and rubbing (amongst others). The knock-on effects of increased levels of the above parameters are often not thought of, such as a reduction in the amount of dissolved oxygen in the water, prevention of oxygen uptake or an increased susceptibility to subsequent parasitic or bacterial infection(s). It is therefore imperative to ensure you regularly check your water quality, and should be the first thing you check if you notice any unusual behaviour in your fish. If you phone up for any technical advice, it will also be one of the first questions we ask you, so it is best to check this beforehand.

Water quality is important in every pond, be it a small garden pond or an enormous pond of champion koi. Following the steps outlined in each section of this article, testing your water quality regulkarly with a multi-test kit and using a tap water dechlorinator for all water changes can help you prevent many problems in your pond, and will reduce the susceptibility of your fish to parasites and bacteria. It is important to remember that all fish are susceptible to illness, stress or toxicity caused by poor water quality, no matter their species or age, and it is up to you to care for them. Maintaining good water quality is the key to fish health, and will allow you to enjoy your fish for many years to come.

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