Proper koi fish care is an ongoing process (like many things in life). There will certainly be a lot of differing opinions on the best water conditions for koi but one thing remains constant: poor water quality leads to a host of other health problems that are certainly avoidable with proper care.  Because stress originates from poor water quality, follow these guidelines for healthy water and healthy koi.  As a preview this article will cover topics like:

  • Proper Pond Salt Levels
  • Proper Koi Fish Care
  • Ideal Koi Water Temperatures for the Pond
  • Best Acid/Base Balance
  • etc.

Dissolved Oxygen

Oxygen levels should be at a minimum of 5.0 mg/L for koi.  In the fish world, there is some variation with the tolerable level of dissolved oxygen in the water but 5.0 is a good baseline.  As a reference, 5.0 mg/L is the minimum for koi and 18 mg/L is the physical maximum that water can hold. 

Just as a reminder though, cold water can hold more dissolved oxygen than warmer water so higher summer temperatures (and overcrowded ponds) will lead to lower dissolved oxygen, which is really when you will want to pay attention to it the most. 

Oxygen becomes dissolved into your koi pond in several ways.  At the very surface there is limited diffusion occurring between the atmosphere and the water and that accounts for only a small amount of dissolved oxygen. 

Turbulence will also agitate the water enough to generate dissolved oxygen in your pond and this often comes in the form of falling water as from a small water fall.  A tried and true and popular way to increase the dissolved oxygen in a koi pond is by an air stone on the bottom of the pond or by spout or fountain shooting water up into the air.

Pond pH (acids & bases)

Okay, back to chemistry class everyone because its time to discuss pH.  As you may recall pH has something to do with acids.  Its all about acidity and alkalinity.  The pH scale is a logarithmic one meaning when your pond jumps from 7 (neutral) to 6, its not simply getting a little bit more acidic, its getting 10 times more acidic. 

Conversely, when your pond tests at 7.5 and then gets increasingly alkaline by jumping to 9.5,  then your pond just got 100 times more alkaline (10 times 10).  So it’s a big deal when your pH changes and can definitely affect your koi in negative ways.  pH, or power of Hydrogen, should range in your pond somewhere between 6.8 and 8.2 but do your best to keep it as stable as possible.  

If you were to test your pond multiple times a day you should notice a daily fluctuation in pH.  So what’s going on?  As part of the natural process of photosynthesis and respiration of aquatic plants and algae they use up dissolved carbon dioxide (that’s a good thing) but, at night, they release it as well (not good).  

The common denominator here is carbon dioxide because of its power to increase the acidity of the water.  Of course, your fish are respiring as well which also adds to the carbon dioxide load in your pond. A good way to add a long-term pH buffer to your pond and protect your fish from pH swings is to add some calcium carbonate material to your pond.

This comes in gravel form or rocks and may be sold under the name “agricultural limestone”.  PH buffering capacity (better described as “KH”) is also why its nice to have concrete-lined ponds -the concrete tends to buffer against pH swings.

Pond KH (carbonate hardness)

No discussion of pH is complete without also talking about KH (or carbonate hardness or alkalinity). Your pond’s KH level will affect how susceptible your pond is to pH fluctuations.  If, for example, your KH reading is particularly low then anything you do to get your pH back to an acceptable range may be short lived as your pond lacks the ability to buffer future pH swings.  

KH should be around 105 ppm with a possible deviation of plus or minus 15 ppm (up or down from 105). Baking soda (sodium bicarbonate or bicarbonate of soda) will increase your pH making it more basic. Conversely, you can add white vinegar to lower, or make more acidic, your pond water.

Pond GH (General Hardness)

GH is really a measurement of the total quantity of dissolved minerals in your water.  Typically, magnesium and calcium are the primary minerals referred to when discussing this parameter. Of all the water parameters to test GH may be the least critical but a good range to shoot for is 60 to 160 ppm though some pond owners will have higher GH readings than that with no issues seen in their fish.

Ammonia, Nitrites, Nitrates

As you may recall, the general cycle of waste in your pond starts with ammonia excreted by your fish then bacteria and oxygen break it down to nitrites which later get broken into nitrates then free nitrogen.  Ammonia, nitrites and nitrates all have the ability to cause health problems for your koi if their levels are not kept in check. 

Along with other health problems, ammonia can essentially burn your fish’s gills and reduce its ability to extract dissolved oxygen from the water.  High nitrites can damage your koi’s kidneys and nervous system and high nitrates, for extended periods, can cause your fish’s immune system to be compromised.  Ammonia and nitrites and the most troublesome when it comes to health problems for your koi but don’t underestimate the power of nitrates over long periods of time.  Here is a guideline for these three water chemistry measurements.


ammonia: levels should be zero. Depending on your pH, you can get away with 0.5ppm (parts per million) or 1 ppm for a short period of time but keep in mind that above a pH of 8.0 ammonia becomes more toxic.


nitrites should be less than 0.25 ppm but ideally you should have a reading of zero.


nitrates: a reading of 20 to 60 ppm is acceptable.


Koi Pond Salt Levels

The use of salt in koi ponds has, for long time, been a tried and true method to deal with various water quality and health problems that arise.  Some of the benefits of salt is that its a cheap way to keep some disease at bay, control algae and may also lower nitrite toxicity.

Additionally, salt plays a part in the osmotic pressure between the fish and the outside aquatic environment.  There is a differential between the solute concentration of the fish’s blood and the fresh water that it swims in so the addition of salt actually lowers that concentration differential and makes it easier on the fish by reducing the amount of work its body has to do. 

A salinity of up to 5 ppt (parts per thousand) or 0.5 % is acceptable.  Testing your pond’s salinity is commonly done with a simple tool called a refractometer.

Proper Koi Pond Water Temperatures

Temperature obviously plays a big role in the overall health of your pond and  it warrants your attention.  Temperature can exacerbate existing problems, especially higher temperatures.  For example, warmer water holds less dissolved oxygen and ammonia can be more toxic.   Although koi can handle temperatures of between 35 and 85 F degrees its best to keep your fish in water that ranges between 65 and 75 F degrees.  And as with pH, try to avoid large temperature swings.

In Conclusion

For first time pond owners its easy to get overwhelmed with all the moving parts of maintaining the best water conditions for your koi pond but like with any complex problem the best way to tackle it is one piece at a time.  Start with a few goldfish or something small and work on dialing in your water chemistry.  Buffers will go a long way to controlling pH fluctuations and a little salt can be a little osmotic relief for your fish (and create an unfriendly environment for parasites).  By volume, freshwater fish are the most popular pet in America so it must not be THAT hard! Good luck!