What and how do I test in my pond water?

Q. What principal water parameters should I check on a day-to-day basis? And what are the recommended levels for a koi pond? Are there any other parameters I should check too? Why is it so important to keep on top of this?

A. There can be so many different chemical and biological reactions taking place in your pond that you need to know exactly what is going on in your pond at any one time. Especially as water quality is by far the single greatest factor that will affect the health of your koi. Furthermore, you can only manage something (or in this case, the different parameters that work together to make up your pond’s water quality) if you actually measure it.

So what are the principle parameters that you should check?

Well this depends largely on the age and maturity of your pond as this will determine how frequently you will need to test your pond. You approach testing water in a new pond in the same way you might approach looking after a brand new car. In its first few weeks out of the show room, you wash it as frequently as you can, and then as time passes and as your idol worship starts to wane, you are happier to accept a little more grime than before. The same can be said for testing a new pond. In the first few weeks you must test frequently, but as time passes and the pond matures, you can afford to test less.

In the early days of a pond, you should test several times a week as you build up your koi collection and your filter matures over the coming months.

By testing the individual parameters, you will soon be able to build up a picture of the current situation in your pond. With a little intuition, you can even use the same data to predict future occurrences and also look back on events that may have occurred historically.

In a more mature pond, you need only concern yourself with 2-3 different tests (pH, nitrite, nitrate and perhaps KH), carried out every week or so. But from day one, you should become familiar with all useful tests, especially as these will help you establish your pond’s unique characteristics.

The seven useful parameters that can be tested to generate a comprehensive understanding of the pond environment can be loosely divided into 2 groups: Biological and Chemical.

Biological Parameters.

These include ammonia, nitrite and nitrate and are grouped as biological parameters because they are most significantly affected or controlled by life within the pond.

Ammonia. Ammonia is toxic (which is why koi excrete it), is colourless and is released from the gills, readily dissolving in pond water. Tests have shown that fish can tolerate higher ammonia levels in more acidic water and as a result may offer a little leeway if an ammonia problem arises in such conditions. However, the only guaranteed way of preventing koi from suffering from ammonia toxicity (whatever the pH) is to keep it at zero.

Nitrite. Nitrite (NO2) is also toxic and is the by-product of the biological (bacterial) breakdown of ammonia. It has a reputation for being more stubborn and persistent than ammonia, with bacteria taking longer to get on top of a nitrite peak. Nitrite levels can often rise out of control for long periods in a new pond, to levels where even a partial water change (30%) does not appear to reduce the problem. If nitrite levels are allowed to become too excessive, then the nitrite itself can become inhibitory to the nitrite-oxidising bacteria, increasing even further the time taken for levels to drop. These bacteria, which include Nitrobacter species, break the nitrite down into less toxic nitrate. Just like ammonia, the desirable nitrite level is zero.

Nitrate: Nitrate is the least toxic of the 3 nitrogenous compounds. It can be regarded as the nitrogen bank, where all of the nitrogen from the pond system is deposited. Nitrates will accumulate within a koi pond over time, and can be utilised through plant growth or diluted by a partial water change. You will need to intervene with a partial water change when it rises to 50ppm.

Chemical Parameters.

These include pH, KH, GH and oxygen and are grouped as chemical parameters as their levels are caused by chemical interactions (some of which may be directly related to other biological processes).

pH. pH measures the acidity or alkalinity of pond water. A koi pond’s pH should fall between 7.0 and 9.0, ideally being stable around 7.5 and 8.5. This slightly alkaline pH (7 being neutral) suits the environmental requirements of koi and with a little effort, should be relatively easy and inexpensive to maintain. Your principal aim with your ponds pH is to keep it stable.

KH. KH is a measure of the soluble carbonate/bicarbonate ions that act as a buffer to maintain a stable pH. A medium to high KH shows that the pH is not likely to fluctuate and is therefore the most desirable. This can be maintained by simply adding a mesh bag of limestone chippings or crushed shell in your filter.

GH. GH measures the general hardness of pond water, particularly the hardness forming ions of calcium and magnesium. The GH should be medium to high in order to match the conditions that are preferred by the koi physiology.

Oxygen. Oxygen, like all of the preceding parameters is required at a minimum level. It dissolves in water and can be easily added using diffused aeration, venturis, moving water or through plant photosynthesis (although this is less common in your koi pond, unless it is suffering from blanketweed). Water will hold less oxygen in warmer water (just when koi and bacteria require more) and if a pond is excessively planted, may well suffer from dawn depletion as DO levels drop at night through excessive plant respiration. DO must be at least 5mg/l.

Boxout: 10 top tips for maintaining a stable water chemistry

  1. Test regularly.
  2. Build your pond as large as possible in the first place. ‘The solution to pollution is dilution’
  3. Carry out frequent smaller water changes than a few larger ones.
  4. Add a source of calcium carbonate in your pond/filter to keep pH and KH stable and within acceptable limits.
  5. When starting a new pond, add fish gradually and test regularly.
  6. If you detect an unhealthy or surprise increase in ammonia or nitrite, stop feeding, carryout sufficient water changes to bring it down to zero.
  7. To avoid the risk of low DO, ensure your pond enjoys several reliable sources of aeration. Remember – you can’t over-aerate.
  8. Keep on top of solids accumulation in your filter by regular cleaning of the mechanical media / chamber
  9. Endeavour to keep your filter running through the winter to avoid any NPS-type water quality issues in spring
  10. Ensure that any new water is passed through a purifier or is treated with a tap water conditioner after a water change.


Boxout 2:

Main, regularly checked parameters.


Parameter Ideal value
pH Stable, between 7.5 and 8.5
ammonia zero
nitrite zero
nitrate Less than 50ppm

Kill blanketweed and string algae.