"I never test my pond water " is a comment we often hear from our customers. Is this justified, or is water testing actually an important part of enjoying your beautiful pond in the long run? We'll try to give you a clear answer.

First of all, it's important to know that your pond water consists of a number of important parameters, which we'll briefly summarize here:

  • PH: This is the acidity of your water
  • GH: This is the total hardness of your pond water (ideally between 10 and 12)
  • KH: This is the carbonate hardness of your pond water (ideally this is 8)

pH value of pond water

First, we will try to explain to you what pH value is and why this is an important parameter.

The pH value indicates the number of free hydrogen (H+) ions. The number of H+ ions determines the acidity of your water on a scale of 0-14. Water (H2O) is composed of H+ and OH- ions, and the ratio of free H+ ions to OH- ions is the pH value. The higher the number of H+ ions, the more acidic your pond water, and conversely, the more OH- ions, the more alkaline your pond water. Hydrochloric acid has a pH value of 0, and sodium hydroxide has a pH value of 14. At a pH of 7, your pond water is neutral; this is the case with pure, demineralized water. Pond water with a pH of 6 (10-6 H+ ions) is 10 times more acidic than pond water with a pH of 7 (10-7 H+ ions). The pH value is expressed in a negatively progressive series. In both a pond and aquarium the pH value continuously decreases as a result of natural biological processes.

When is my pH value correct?

The pH value in your pond is ideally between 7 and 8.5. If you have a koi pond or a pond with only other cyprinids, then the pH value is fine if it is between 8.0 and 8.5. Assuming a neutral pH value of 7-8.5 for fish, a value of 6 may not seem like much of a problem. However, a pH value of 6 is already 10 times more acidic than a pH of 7. If your pond water has a pH value of 5, then it is 100 times more acidic than a normal pH of 7. You can imagine that your fish will not be happy with you!

Can an unstable pH value harm my pond and fish?

When the pH is too low or too high, or fluctuates significantly, your fish become stressed. Their gills and fins become bloodshot, and their mucous membranes can become damaged. If your fish are stressed, they quickly become ill because their resistance is drastically reduced, and it can ultimately kill them. Beneficial bacteria in your pond and your filter will also be less effective due to poor pH levels, causing the pH level to deteriorate even faster. It is therefore very important to keep a close eye on the pH value of the pond.

However, you can prevent a low pH value

In any case, it's crucial that your pond water has sufficient hardness. The GH and KH must therefore be sufficiently high. Of course, factors like good biological filtration, bacterial balance, and aeration also play a role. Furthermore, it's wise to keep your fish moderate and to have plenty of oxygenating plants.


GH value pond

Now that we have explained the pH, we will continue with explaining the GH value of your pond.

The GH value indicates how many minerals are dissolved in your pond water. Important minerals that should be present in your pond water are calcium and magnesium.

Minerals are very important for your pond for two reasons:

  1. minerals form the means of transport for CO2 in your pond;
  2. minerals are nutritional salts for oxygenating plants .

For healthy plant growth, a GH value between 10 and 13 is ideal. A lower value will stagnate your plant growth, while a higher value can easily cause limescale deposits on your oxygenating plants. Tap water in the Netherlands and Belgium has a hardness between 0 and 15, depending on the region (water companies often list the hardness of their tap water on their websites). Rainwater is extremely soft and therefore unsuitable for filling a pond.

Oxygen-producing plants extract minerals from the water to grow. Incoming rainwater contains no minerals. This means the GH value of your pond can only decrease .

In every pond, the hardness eventually becomes too low to allow oxygen-producing plants to grow. Therefore, the GH value must be periodically adjusted , otherwise the pond will become cloudy.

Ensure the GH value is brought to a good level (at least 7) ​​at the start of the growing season . After the winter, the hardness of the pond water usually decreases significantly due to precipitation. The GH value will also decrease over the course of the growing season as aquatic plants extract minerals from the water. Raising the GH value is also recommended in this case.

Are you still getting it? There's a lot to it, right? We hope we still have your attention, because this is important.

KH value pond water

Finally, we will discuss the KH value

The carbonate hardness indicates how much carbonate is dissolved in your pond water. A carbonate is a chemical compound of calcium/magnesium and CO2 . This CO2 component is crucial: your oxygen-producing plants need this CO2 for their growth. If the carbonate hardness is sufficiently high , there is essentially a reserve of CO2 present in your pond. In principle, sufficient CO2 is produced by the bacteria in a healthy pond, but for various reasons, production by the bacteria and consumption by the oxygen-producing plants can sometimes differ. sufficiently high carbonate hardness can absorb these differences. This is also referred to as the buffering effect.

New ponds almost always lack CO2 because there are relatively few bacteria present. Adding carbonate is a good way to get through the critical start-up phase. A good KH value is at least 7 and no more than 8.5.

So testing the water in your pond is really useful!

To continue enjoying a healthy, clear pond and healthy fish, you'll need to test your pond water. Fortunately, with the right tools, this is a breeze. It only takes a few minutes and is very convenient.