koi fish swimming in a pond

As a koi keeper, you only have so much control over your koi. However, what you do have control over is their environment—the water. Wiser heads have said that, first and foremost, koi owners are waterkeepers. Water that is well maintained and fastidiously monitored is the surest pathway to healthy and happy koi.

There is more to water than one oxygen molecule and two hydrogen molecules. Well, actually, no, there isn’t. But there is more to ensuring that it is the best molecular compound it can be.

Many a koi keeper will look upon a pristine, crystal-clear pond and think that all is well. However, clear water DOES NOT mean healthy water. There can be harmful, or even fatal, compounds in a clear pond, while water that seems a little off-color can be completely healthy.

To provide that ideal aqueous environment for your koi, it is essential to know all the important facets of water, how they impact your koi, how to recognize when they are out of balance and all the ideal and acceptable ranges.

Nitrogen Cycle

The importance of the nitrogen cycle cannot be understated, especially for a koi keeper who manages a closed water system. A nitrogen cycle that is working in harmony is a wonderful thing that will sustain all forms of aquatic life, both fish and plant. When it is out of sync, however, only bad things happen. Death being the worst of all.KOI nitrogen cycle v2

Ammonia (NH3)

Ammonia is the stuff of nightmares for koi keepers. In fact, ammonia poisoning ranks as one of  the leading causes of fish death.

NH3 (Or Ammonia) is a natural by-product excreted by koi through their gills and their waste. It is also released by any decomposing organics (food, leaves, etc.) left in your pond.

And ammonia is deadly poisonous.

Even in small concentrations, it is enough to kill a koi population. If it doesn’t do it directly, ammonia opens the door to opportunistic attacks by weakening your koi’s immune defenses.

The ideal ammonia concentration is as close to 0.0 as possible. Even an amount as little as 0.15 ppm is enough to poison your koi. It damages the sensitive and delicate gill tissues, reducing their efficacy and allowing for bacterial or parasitic infection. Ammonia also damages the skin and slime coat, which is another conduit for bugs to attack your already weakened koi.

Signs of Elevated Ammonia

(Note: Many of the symptoms associated with elevated nitrogen-based compounds in the cycle will be similar)

  • Irritation
  • Gill and/or fin burns (looks like they are bleeding)
  • Lethargy
  • Loss of appetite
  • Fin clamping
  • Gasping at the pond surface
  • Lying at the bottom
  • Excessive slime coating
  • Red streaks on fins and/or skin

Treating High Ammonia Levels

Testing water levels frequently is always a good defense against high ammonia levels. Tests will let any koi keeper know what they are dealing with and how severe the problem is. Once you’ve identified your pond is suffering from high ammonia levels, you can:

  • Lower the pH
  • Feed smaller amounts
  • Make a big water change (up to 50%)

Nitrite (NO2−)

Nitrite is another poisonous chemical compound for koi fish. Concentrations as low as 0.15 ppm can result in Brown Blood Disease. Once nitrites enter your koi’s bloodstream, they bond with the hemoglobin in red blood cells forming methemoglobin, which turns red blood cells a chocolatey color—hence the disease name.

Without the necessary amount of hemoglobin (which is responsible for absorbing the oxygen from the water and transporting it through the koi’s body), the koi essentially suffocate even with high levels of oxygen in the water.   

Signs of Elevated Nitrite

koi gasping at the surface for airAgain, testing water levels will help you identify if your pond is suffering from high nitrite levels. Other signs include:

  • Flashing (rubbing against the pond walls or bottom)
  • Fin clamping
  • Rapid gill movement
  • Gasping at the pond surface
  • Lying at the bottom
  • Fish appear pale or dark

Treating High Nitrite Levels

If you find you have high nitrite levels, you can:

  • Lower the pH
  • Make a water change (upwards of 25%)
  • Ensure adequate oxygen levels (for the koi and the Nitrobacter to be able to oxidize the nitrite)
  • Add salt to 0.3% salinity (this will kill some of your aquatic plants) in extreme circumstances

Nitrate (NO3−)

Nitrates are the least harmful of the nitrogen compounds in your pond. That doesn’t mean that they can’t still be dangerous in large enough concentrations. Nitrate levels over 60 ppm should trigger water changes. Levels over 120 ppm can cause severe damage to a koi’s fins.

Aquatic plants (and algae) are big fans of nitrate. They consume it and then release oxygen back into the water.

Signs of Elevated Nitrate

Symptoms of elevated nitrate levels include:

  • Dullness
  • Lethargy
  • Loss of appetite
  • Loss of equilibrium
  • Rapid gill movement

Treating High Nitrate Levels

If you find you have high nitrate levels, you can:

  • Make a water change (upwards of 10-25%)
  • Reduce feedings


Nitrosomonas are the first line of defense against ammonia. It is an aerobic, rod-shaped, gram-negative nitrifying bacteria that oxidize ammonia as part of its own metabolic and reproductive process. In other words, it eats the ammonia, burns it with oxygen and produces nitrites. Nitrates are still extremely harmful to koi and need to be removed. This is where nitrobacter come in.


Nitrobacter is the next stage of beneficial bacteria in the nitrogen cycle. Nitrobacter is another aerobic, rod- or pear-shaped, gram-negative nitrifying bacteria. Only, this beneficial bacteria uses nitrites for energy production and converts them to the much less dangerous nitrates. These nitrates are then used by aquatic plants or algae.

If you have a new pond, don’t add your best or most expensive koi immediately. To feed the nitrogen cycle consider adding some inexpensive “canary” koi in there that you aren’t attached to or haven’t cost you a lot to purchase. Wait up to 6 weeks before adding your best koi to allow for the beneficial bacteria colonizing your filters to get a handle on the ammonia in the pond and start breaking it down.

Oxygen Levels

Oxygen in fish, as in humans, is key to life. Your koi may go about getting it a little differently, but they need it as much as we do. To keep your koi well oxygenated, the dissolved oxygen level in the water should be anywhere from 7-10 mg/L.

Aquatic plants, although adding oxygen through photosynthesis during the day, require it at night for respiration. So make sure your pumps and aerators can handle new growth in the warmer months.

To raise oxygen levels in a pinch, breaking the surface with a hose and sprinkler will add some oxygen at the top level of the pond.

Also, when adding water as part of water changes, be advised that the process generally lowers oxygen levels in the pond as it is less oxygenated.

Signs of Low Dissolved Oxygen

If your pond is suffering from low levels of dissolved oxygen, you might see these signs:

  • Fish concentrated around oxygen sources (waterfalls, aerators)
  • Koi gasping at the surface

Side Effects

Low dissolved oxygen has many effects on koi, including:

  • Slow or stunted growth
  • Chronic illness, such as bacterial infections
  • Repeated parasite infections
  • Reduced reproductive capabilities
  • Death


Temperature and Dissolved Oxygen concentration chart for koi ponds
Courtesy: http://www.koihealth.info

Higher water temperatures will increase a koi’s overall energy and activity. The higher the temperature, the more active (and hungry) your koi will be. Colder water temperatures have the opposite effect.

Temperatures also affect the amount of dissolved oxygen in the pond water. Higher water temperatures can hold less dissolved oxygen, meaning that more oxygen will need to be added to the water column in the summer months to keep par with the beneficial bacteria.

Higher temperatures also cause organic materials to decompose quicker, which can add higher levels of harmful nitrogen compounds to the water.

Shade and/or floating water plants can help to keep water temperatures down by minimizing direct sunlight.


The potential of hydrogen is another important consideration for waterkeepers. pH is a measure of how acidic or alkaline your water is. Measured on a scale of 1-14, the higher the number the more basic (more hydroxyl ions) the water is, and the lower the number the more acidic (more hydrogen ions) it is. The value of 7 is the balance (same number of hydroxyl and hydrogen ions) between basic and acidic.

All biological activity in a koi pond will reduce the pH (making it more acidic), so some daily fluctuations are normal. It is when there are dangerous and drastic swings that action needs to be taken.

The water that you use for water changes may naturally be hard or soft water. Hard water has higher amounts of minerals (usually calcium) that acts as a buffer, raising the pH to make it more alkaline. Soft water, conversely, lacks those same buffering minerals and so has a lower, more acidic pH. It is a good idea to test your water and determine which (if either) you have

Ideally, pond water should be as close to 7 as possible. A range of 6-8-8.2 is considered safe. Acidic water will burn the koi’s skin, while basic water will irritate and chaff the skin.

Why is pH important?

A single-digit change in either direction from neutral (7) represents a 10x increase in acidity or alkalinity.

Koi have the ability to regulate the pH in their blood both up and down to accommodate swings, but it takes time. Swings that are too big or too sudden can result in death for your koi through acidosis or alkalosis.

Signs of Alkalosis (pH is too high) and Acidosis (pH is too low)

Red streaked fins on a koiKeep an eye out for the following signs your pond’s pH levels might be off:

  • Blood-streaked fins
  • Loss of appetite
  • Excess mucus production
  • Lethargy
  • Laying at the bottom
  • Gasping at the surface

Alkalosis is more pronounced and harder to reverse than acidosis, which can be combated by raising the pH.

Adjusting pH

There are several ways to adjust the pH levels of a pond.

  • Adding baking soda (NaHCO3) is the quickest way to raise your pond’s pH
  • Oyster shells will help to raise the pH of naturally soft water
  • Adding vinegar (which is acidic) is a quick way to reduce a pH that is too high
  • Making water changes (this readjusts the buffering capacity by removing older water that has lost that capacity)


KH (carbonate hardness) is the measurement of bicarbonates and carbonates in pond water, and it is another facet of waterkeeping that should be given some recognition and consideration. It is another component that will influence the stress on your koi’s system.

KH is sometimes referred to as total alkalinity. As alkalinity is a measurement of your pond water’s ability to absorb and neutralize acids (buffering), the higher the KH value the less likely the water is to experience a pH crash. In other words, favorable KH levels help stabilize your water.

Raising KH

In order to raise KH levels in your pond, you can

  • Add baking soda (sodium bicarbonate )
  • Increase aeration (via air stone or similar means) to bubble off carbon dioxide (CO2)
  • Add oyster shells

Lowering KH

In order to lower KH, you can

  • Inject carbon dioxide (CO2)
  • Use reverse osmosis (RO) water (water that is stripped of everything including KH to give pure H2O) mixed with tap water until desired kH is achieved.

Dissolved Organic Carbon

White foam on the surface of your pond is a good indicator that there are too many decaying organics in your pond. This decomposition causes excess nutrients (nitrates) in your pond that can provide sustenance for a lot of things that you don’t need if you are looking to keep healthy water in your pond.

High DOCs can impact your koi’s growth. Because of their organic acid structure, DOCs can also reduce pH affecting pond water parameters. DOCs can also place higher demands on the oxygen in your pond.

Signs of High DOCs

Foam on top of a koi pondYou may have high DOCs if you observe the following signs:

  • Foam on the water’s surface
  • Algae blooms
  • Ulcers on your koi
  • Lethargy
  • Appetite loss

Treating High DOC Levels

If you have high levels of dissolved organic compounds, you can try the following:

  • Water changes
  • Add activated carbon
  • Adjust your fish load
  • Keep the pond debris free
  • Slow down feeding or reduce the amount of food
  • Increase filtration
  • Invest in a foam fractionator

Regular water changes

It is generally considered standard operating procedure to change anywhere from 10-25% of the water in a pond every week.

Depending on the temperature, season and feeding schedule, water change amounts and schedules of water changes can be adjusted. Pondinformer.com offers a chart for water change frequency and volume.

Benefits Of Water Changes

  • Removing a certain amount of water will help to reintroduce some trace minerals that have been depleted, as well as flush out the waste and by-products that have accumulated through normal biological action (as well as some stuff that might have crept into your pond without your knowing).
  • It removes naturally-produced pheromones that can inhibit growth and increase stress.
  • It lowers the incidence of illness, infections, and parasite infestations.

Water changes are also imperative if the pond has been treated for bacterial, parasitic or fungal outbreaks.

Smaller, consistent water changes of 10-20% place less stress on your koi. Whereas large, rapid water changes should be reserved for emergencies.

The nutrient-rich water you remove can be beneficial for plants and some keepers will irrigate their plant beds, vegetable gardens or yards with it.


Both chlorine and chloramine are dangerous to koi. Chloramine is often added to water by cities and municipalities in addition to chlorine. Sometimes the two will be used interchangeably. Chloramine also contains ammonia, which brings its own danger.

In your koi, both of these additives will block the transfer of oxygen from the water to the gill tissue. They will also cause necrosis in the gill tissues. As an oxidizer, they irritate the skin and affect the slime coa

Chlorine and chloramine will also kill off beneficial bacteria in your filter. Even in small quantities, they are harmful. At 0.35 ppm, your fish will die.

Lower pH values also increase chlorine’s toxicity. Additionally, phenols (released by some organic wastes) combine with chlorine to make for a toxic cocktail.

When performing water changes it is important to add dechlor (sodium thiosulfate) to the water to neutralize the chlorine.

Often times, chlorine and chloramine compounds will reach dangerous levels in pond water when the hose is left running after a water change or as a top off for evaporation. Putting a timer on the faucet or hose can be a useful addition to the water change arsenal for absent-minded koi keepers.

Signs of Elevated Chlorine/Chloramine

The following are signs of elevated chlorine or chloramine levels:

  • Paleness
  • Excess or reduced mucus
  • Blotchy redness (hyperemia)
  • Gasping at the surface
  • Erratic swimming

Treating High Chlorine/Chloramine Levels

To treat high chlorine or chloramine levels, you can

  • Add sodium thiosulfate (be careful to use the accurate dosage)
  • Increase aeration (it will help to bubble out the chlorine quicker)
  • If possible, reduce water temperature (to increase dissolved oxygen)

So, as you can tell, there is a lot more to water than meets the eye. Keeping your koi in healthy and life-giving water is a constant vigil. But the benefits for your own peace of mind and your koi are immense.

6 responses

    1. We would recommend testing your water with a test kit to ensure all parameters are within acceptable readings.

  1. Mercedes Simon :

    I was thinking of investing on a reverse osmosis system to aid with ammonia, but also water changes. Currently when I do water changes I use a hose attachment filter to get better water. Thoughts?

    1. rosaline :

      We apologize, as we are not experts on the topic of reverse osmosis.

  2. At what level of kh is considered harmful to koi. I have read many conflicting information sights about the subject and what it does to koi?

    1. Hi Steve, We are not experts on this topic. However, it is believed that KH levels are not harmful to Koi. The ideal KH level for ponds can range from 75 to 200 mg/L (ppm) CaCO3.

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