Koi Pond Water Quality

If you observe a fish exhibiting behaviors that you have not seen before or you notice changes in your pond water.


For Example:

  • Laying on the bottom of the pond
  • Difficulty swimming
  • Staying away from other fish
  • Not eating for an extended period of time
  • Gasping for air
  • Flashing, excessive scratching
  • Laying over on it’s side
  • Pond water has been “foamy” or taken on a yellowish tint

If any of these conditions are present in your pond you need to check your water quality!


To do this we recommend that you purchase a quality water test kit. They are available at most quality pet shops and over the internet. These kits usually come with several graduated test tubes and various squeeze bottles containing different solutions of reactive test agents. Also supplied with these kits are instructions and color keys so that you can interpret the results of each test.


The items these kits test for are:


  • Ammonia
  • Nitrite
  • Nitrate
  • PH

Here are the numbers you should strive for in your pond for happy and healthy fish:

Ammonia is the most toxic and will cause death in very low concentrations (0.15 ppm or 0.15 mg/liter). A good pond with an active biologic filter should read under 0.1 mg/liter. Acceptable level = 0

Nitrite over 0.1 mg/l, according to your test results, you should do a partial water exchange. Acceptable level = 0

Nitrate if this level is over 20 mg/l you should do a partial water exchange or add the appropriate amount of AmQuell+©. Optimum is less then 60 ppm however the closer to 0 ppm is best.

PH indicates the ratio of Hydrogen ions (acidic) to Hydroxyl ions (alkaline) on a logarithmic scale from 0 (pure Acid) to 14 (pure alkaline). Pure water is 7.0, meaning that there is an equal balance of hydrogen ions and hydroxyl ions. Most tap water is around 7.4 to 7.6, which is perfect for Koi, as they do best in water at 7.2 - 8.0. Koi can actually tolerate a wide range of PH, from 6.5 all the way up to 9.0, but they cannot tolerate a rapid change, no more than 0.2 per hour. Acceptable rang for happy Koi is 7.4 - 8.4.

Temperature is often viewed by pond owners as a guide to feeding more than it is as a health issue for their Koi. Water temperature should be monitored for both daily swings and seasonal extremes. Temperature affects dissolved oxygen levels, respiration, metabolic rate, PH balance, free ammonia/ionized ammonia ratio and osmo-regulation. Koi can tolerate a broad rang of temperatures, from ponds that are iced over to water up to 90 degrees F, better than they can tolerate sudden shifts temperatures. If you have a shallow pond (less than 2 feet deep) in full summer sun with cool summer nights, the pond temperature may be changing by more than 4 degrees F. An hour causing stress to your fish. Greater splashing of the water (with a water fall) and shading may help control the temperature fluctuation. If your pond is subject to stressful temp changes, a 0.1% solution of sea salt containing Calcium, potassium, sodium and trace elements will reduce the stress as it aids the Koi’s osmo-regulation. As with PH, do not drastically alter the ponds’ temperature by, for instance, adding ice in the summer! Do it slowly. Koi can tolerate a low to high temp change better than they can a high to low change.

Water Hardness consists of two elements, permanent or general hardness and temporary or carbonate/bicarbonate hardness. Koi do better in hard water because of the relation of salt within their bodies to the dissolved salts in the pond water. In soft water, the difference in salt concentrations means the Koi have to work harder, through the process of osmo-regulation, to prevent the salts within their bodies from diffusing out through their gill membranes. Harder water allows the Koi to ease up on osmo-regulation and therefore reduces stress. As mentioned above, bicarbonate ions buffer the water, reducing the PH shifts, another cause of stress in Koi. Koi do well in carbonate hardness of 150-300 mg/liter or 9-18 degrees dH. In most Koi ponds the water is too soft due to the fact that there is no natural mud bottom that leaches minerals into the water. Marine salt and sodium bicarbonate increase hardness, and will also cause pH to go up. A permanent salt solution of 0.1% is beneficial to Koi, and works out to about eight pounds per 1,000 gallons. Check your pH if you add salt, and do not use table salt - the salt used to make salt water aquariums is the best. Salt will not evaporate out, and needs to be replaced only if water is drained from the pond.

Dissolved oxygen is usually only a warm weather concern, as it is associated with water temperature and algae. However, the larger the fish, the greater its oxygen demand - low oxygen levels will stress and kill your biggest fish. Ponds that have been safe may become unsafe as your fish grow larger. The colder the water, the greater its capacity to hold dissolved oxygen. Algae take up oxygen at night, and an algae bloom can cause suffocation in large fish and inhibit the oxidation process of nitrifying bacteria. Also, dying algae and decaying organic material take up oxygen. Testing for dissolved oxygen allows you to determine if your pond has the maximum amount for the temperature of the water. Splashing the water into small droplets with a fountain or waterfalls is best for aeration, although venturi valves on underwater jets and air compressors also do a good job of oxygenation.

Chlorine and Chloramine should be tested for if your water supply is from any source other than your own well. Chlorine will burn off by itself in a day or so, but chloramine must be broken down and removed chemically. Check with your local water agency to determine whether they add chlorine or chloramine. These chemicals damage the gills and liver, and even in low concentrations can cause stress that ultimately leads to disease. Also, frequently overlooked is the fact that they are added to the water supply to kill bacteria. The beneficial, nitrifying bacteria in your biological can be killed off by chlorine or chloramine in concentrations that do no obvious damage to your fish.

Copper should be tested for if water is supplied to the pond via copper pipes or if coins are thrown in the pond. Copper, in its most toxic free form, will leach into softer water more readily than into hard water.

The third thing you should do is a thorough pond cleaning;

Remove any debris i.e. dead leaves and settled solids from the bottom of the pond, Clean out your filters, Clean out your settling tank, Make sure your bottom drain/s have good water flow, clean out your surface skimmer basket/brushes.

Next you should do a 40% - 50% water change. Use a de-chlorinator like AmQuell +© if you are on city water. This product will Remove/detoxify all of the kinds of toxic nitrogen compounds in the water and all forms of ammonia, ammonium, nitrites & nitrates from the water, including the ammonia in chloramines.

If after a few days the water in your pond checks out to be within normal limits yet your fish are still showing signs of sickness, then we suggest you Isolate the fish, or fishes in question, from the rest of your pond into a “self” contained, (Isolated from the main pond water system), holding tank, with excellent water quality and adequate filtration. This Isolation Tank should also be “salted”.


Real water changes should be undertaken on a regular basis on one of the following schedules

  • 10% every week
  • 20% every two weeks
  • 30% every three weeks

One of the determining factors in setting up your Water exchange schedule is related to the following criteria:

  • Size of your pond (gallons of water)
  • Environmental conditions
  • Fish load (how many fish can live in your pond). One rule of thumb for outdoor pond is there is no rule of thumb! “Fish load” (the number of fish that you can stock in a pond) really all depends on the size of the pond, the type of filter, the size of the pump (it should be capable of “turning Over” the entire volume of water every hour or two, adequate aeration and last but absolutely not least is water quality. Remember the more and larger the fish the more waste they produce! Make sure the mechanics and the bio filter of your pond can handle it!
  • The actual “Water Exchange” is accomplished by pumping out the appropriate amount of water, according to your pond size, fish load and the schedule you are using. Then “spritzing” the water into the pond slowly, this will dissipate a lot of chlorine, but it will not dissipate all of it.

For this, if you are on city water, you need to use a de-chlorinator like AmQuell +©. This product will Remove/detoxify all of the kinds of toxic nitrogen compounds in the water and all forms of ammonia, ammonium, nitrites & nitrates from the water, including the ammonia in chloramines.


Remember folks the vast majority of Koi Ponds are closed eco-systems and except for some rain and the water you put in, there are no other sources of fresh water! And even with a moderate fish load “Fresh Water” doesn’t stay “Fresh” for long!


Failure to do pond water changes, on a regular schedule, allows the accumulation of of these compounds, such as phosphates and proteins which inhibit Koi and pond fish health and growth. Finally, water changes need to be done regularly in order to replenish the trace elements and minerals in the water which Koi fish need to reach their full potential.


Water Quality is by far the most important environmental factor for maintaining health fish. It is also the easiest to maintain. By not maintaining it will cause your prized Koi, Butterfly Koi and Goldfish to exhibit many behaviors related to stress which can look like it is suffering from some malady. 


Water quality must be compatible with the requirements of the fish being held, especially with regard to ammonia, nitrite, nitrate, pH, temperature, dissolved oxygen, hardness, alkalinity, and salinity. Although initial water quality in a system will be determined by water source and water treatment regimes, the long-term water quality in a re circulating system depends on numerous factors. The most important considerations are the source of water in the system, fish load, feeding rates, and bio filter capacity.


Water from the source should be evaluated by an Aquaculture specialist and a water-testing laboratory before a system is established. Water from different sources may have different potential problems that must be addressed. Municipal water may contain chlorine's or chloramines; well water may contain hydrogen sulfide, supersaturated gases (such as nitrogen gas, resulting in gas bubble disease), high carbon dioxide levels, and low oxygen levels, or high dissolved iron levels, (all conditions that can be lethal to fish if not corrected); surface water sources may be high in bacteria or toxic chemicals resulting from run off. 


The most common water quality problems in re circulating systems are toxic levels of ammonia or nitrite caused by imbalances between the capacity of the bio filter and the fish load and feeding rates. This problem often occurs during the start up of a system, although it may occur at any time. The bacteria in the bio filter can require three to eight weeks to cycle (i.e., become established) at 25–27°C (77–81°F) and even more time may be required at cooler temperatures (see UF/IFAS Fact Sheet FA-16 Ammonia). Aqua culturists will often begin this cycling process prior to the addition of fish by one of the following methods:


  • adding ammonia directly into the system
  • adding a species of fish that is more tolerant of the initial high ammonia and nitrite levels before adding the final species to be held; a caveat, of course, is that these fish have been checked for potential pathogens they may bring to the syste or
  • seeding the system/bio filter with bacteria from a “healthy” established system or with bacteria from a reputable commercial source

In established filters, toxic levels of ammonia and nitrite may result from overfeeding, crowding, or inefficient removal of solids (such as feces and uneaten food), resulting in breakdown of large quantities of proteins into ammonia. However, in addition to problems caused by source water issues, ammonia, and nitrite (described above), problems can also result from changes in water quality parameters that were previously acceptable. Parameters that can change over time in a system include dissolved oxygen (DO, decreases), alkalinity (decreases), carbon dioxide (can increase), and pH (decreases). 


Low DO can occur during operation of a system as the result of many different causes. Some of these include: high stocking densities, inadequate water flow, inadequate aeration, high organic loads in the system that lead to large numbers of bacteria in addition to those in the bio filter, high feeding rates or the use of certain chemicals such as formalin (see UF/IFAS Fact Sheet VM-77 Use of Formalin to Control Fish Parasites). Re circulating systems with little loss or addition of water often undergo gradual pH drops, a result of gradual acid addition and alkalinity reduction in the system. The bacteria in the bio filter produce acid (H+) as a by-product of nitrification (the process occurring in the bio filter that converts toxic ammonia and nitrite into much less toxic nitrate). In other words, as ammonia is converted to nitrite and nitrite is converted to nitrate, Hydrogen ions (H+) are released into the water. This acid then uses up the carbonates and bicarbonates by combining with them (which essentially neutralize them), causing a reduction in alkalinity. Drops in alkalinity may also result in ammonia and nitrite spikes because, in addition to oxygen, ammonia, and nitrite, the bio filter bacteria require the bicarbonate portion of alkalinity for survival and growth.


Acids also come from the decomposition of uneaten food and fish wastes and from carbon dioxide released by the fish and bacteria in the water. All of these acid additions react with the bicarbonates and carbonates, removing them from the water and also leading to drops in alkalinity. In addition to alkalinity's importance for bio filtration, alkalinity is very important as a pH buffer. Components of alkalinity (especially carbonates and bicarbonates), as described above, neutralize acids, and help to prevent major drops in pH that would otherwise occur. However, if the alkalinity does reach a critically low level, the pH of the water will drop rapidly and have detrimental effects on the fish and bio filter. Minimum alkalinity levels recommended for good bio filtration range from 100–180 ppm (in particular, the levels of bicarbonate and carbonate ions). Holding species that thrive in lower pH and lower alkalinity, such as discus, can make maintenance of a balanced bio filter much more challenging.

Some management options to prevent this drop in alkalinity and pH and the potential rise in ammonia and nitrite include:

  • routine (once a week to once a month, depending upon stocking density) measurement of alkalinity and pH, in addition to other parameters mentioned previously
  • partial water changes on a routine basis (amount dependent upon drops in alkalinity and pH over time) as long as the source water has an adequate amount of bicarbonate/carbonate (100 mg/L or more); and
  • adding sodium bicarbonate (baking soda) buffer as needed.

Other important water quality parameters that may require monitoring include hardness, salinity, organics, and conductivity. These parameters may increase over time in a system that does not undergo routine water changes, but is only “topped off.” In such a situation these parameters may reach levels that are not desirable for maintenance or reproduction of some fish species. 


Heavy metals, such as copper, zinc, and lead, are also toxic to fish and may be present in the water source. If they are, methods to remove them should be considered (consult an Aquaculture or water filtration specialist), and system levels should be checked regularly. Additionally, heavy metals (such as copper or zinc) may be part of the hardware of the system (though this is highly undesirable) and may begin to leach into the water as the pH becomes more acidic. Check with an Aquaculture specialist for the best construction materials.