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The quality of the water is crucial for the balance in the pond. In addition to the total hardness, the carbonate hardness (KH) is an important aspect that must be considered. The KH value has a decisive effect on the pH value in the pond water. If the value is too high or too low, many processes in the garden pond run incorrectly or not at all. For this reason, it is important to know how to lower or raise the carb level.

carbonate hardness

Before you apply measures to lower or increase carbonate hardness, you need to know about the recommended values. The carbonate hardness is the value of the dissolved minerals in the water. These include:

  • calcium
  • magnesium
  • beryllium
  • strontium
  • barium

This makes it possible to specify the acid-binding capacity that currently prevails in the pond and has a corresponding effect on the biological processes in the small ecosystem. It can happen that the carbon hardness value of the pond water is too high or too low for a variety of reasons. This has a strong impact on the buffering effect, which supports the natural cleaning of the pond. The following list gives you an overview of the individual KH values:

  • too low: below 5° dKH
  • good: 5° dKH to 14° dKH
  • optimal: 6° dKH to 10° dKH
  • too high: over 14° dKH

Please note that the hardness of the pond water must differ from that of the aquarium water. This is because aquariums are not exposed to the following influences that have a noticeable effect on the value:

  • weather
  • wild pond dwellers
  • wild plants
  • organic waste (example: leaves)
  • Sun
  • seasons

The weather in particular ensures that the value changes almost every day. For this reason, the value remains much more stable in an aquarium, while you have to pay more attention to it in the pond.

notice: In order to measure the KH value of your pond water, there are various tests that you can buy in stores. The droplet test should enable the most accurate measurement, but test strips that you can use on your own are just as effective.

Lower KH value

A KH value that is too high is much more rare than one that is too low. However, it cannot be ruled out that your pond may have problems due to excessive carbonate hardness. If the carbonate hardness is too high, the total hardness increases together with the concentration of CO₂ within the water, which has a negative effect on the pond equipment such as pumps or filters in the long term. In addition, if the CO₂ level is too high, the self-cleaning of the pond no longer works, as it is deposited in the water and cannot escape. Causes for a too high KH value in the pond water are:

  • pond care products
  • mineral stones
  • lime water

Hard water is often observed with a too high carbonate hardness. For example, if you have added hard water at regular intervals, it increases the KH significantly. There are three methods to lower the KH again.

tip: It is absolutely necessary not to add peat to the pond, as this does not actually soften the water, but only has the effect of softening for a short time. In itself, peat only releases acids into the water, which lowers the pH, but not really the KH.

supply water

The supply of fresh water, which causes softening, is particularly easy and immediately noticeable. With this variant, you should use the following types of water to reduce the high lime content that is present in most cases:

  • rainwater
  • well water
  • filtered water

Always add a little more fresh water to the pond circuit over a period of several days so that the carbonate hardness can adjust. Check the KH of the pond water from time to time to follow the course. Since the KH value can drop quite quickly, you have to stop early, otherwise the carbonate content will drop sharply. For this reason, check the pond water for KH at regular intervals.

water change

If the water in the pond is clearly too hard, you can also carry out a water change. Since the water binds the carbonate, you can quickly and effectively lower the value by replacing it. In most cases, a complete water change is not even necessary for this, since fresh water also has a positive effect on the KH value, as described above. Also use one of the water types mentioned above for the water change in order to be able to benefit from the low-lime water for a long time.

UV-C lamps

Another method is the use of UV-C lamps. These are UV lamps that, with their wavelength of 315 to 280 nm, are able to soften the water, which at the same time affects the carbonate hardness. Depending on the size of the pond, you need a different number of lamps that shine directly onto the water surface. The following points should be observed when using the lamps:

  • low wattage
  • effective pond filter system
  • use permanently

Again, it is important to measure the water values regularly to find out the current hardness. You can use this to find out when you no longer need the lamps. Since you have to leave the lamps switched on permanently for this method, it eats into your wallet quite a bit. For this reason, you should choose lamps with a low wattage indicated in the list, as they use less energy.

Increase KH value

If the carbonate hardness is too low, you must increase it in order to be able to prevent strong pH value fluctuations. If the KH is too low, there will quickly be an imbalance between carbonic acid and calcium in the pond water, which increases toxicity. Above all, the concentration of nitrite and ammonium should be mentioned here. The water stagnates, which hinders the self-cleaning of the pond and can severely affect the pond inhabitants. Possible causes for this problem can be:

  • persistent rains
  • lack of fresh water
  • Missed water change

Simply adding fresh water to the pond or doing a water change can solve the problem on its own. However, this is not always the case and so you have to use other methods, especially if a water change is not possible, for example. Two methods are available for this.

notice: It is absolutely necessary to refrain from adding rainwater, as this would further reduce the KH value. Because of this, many people in rainy areas struggle with this problem.

calcium carbonate

Calcium carbonate (CaCO₃) has established itself as an ideal means of increasing the KH value and can be used without any problems. This agent is carbonate of lime, which is added to the water in certain quantities and thus has a corresponding effect on the KH. You have to use the calcium carbonate over a long period of time because it works slowly, but at the same time it is the gentlest option for your garden pond. CaCO₃ is available on the market in the following forms:

  • CaCO₃ powder
  • shell grit
  • chalk lime

These have a high CaCO₃ content and are available for a price of one to 15 euros per kilogram, depending on the product in question. Make sure that you grind the sediment as finely as possible, because this is the only way it can work in the water. It is applied in the following way:

  • Fill CaCO₃ in multi-chamber filter
  • alternatively fill in stream
  • check water values daily
  • refill if necessary

Hardening the KH value in this way can take a little longer, since the pond can only increase by a maximum of 1° dKH per day. If you use too much calcium carbonate, the carbonate hardness in the pond water increases too quickly and can also have a negative effect on it. For this reason, check the water values daily until you have reached a good KH value. Patience pays off with this method.

Water inlet of a garden pond

tip: If you are the proud owner of a stream, you can place limestone in it to harden the water. The water dissolves the calcium carbonate from the limestone over time and in this way regulates the KH in the pond excellently and with little effort.

conditioning agent

As an alternative to adding some form of calcium carbonate, you can use ready-made conditioners from specialist retailers or hardware stores. These are agents that you add to the water according to the manufacturer's instructions to increase the KH within a short time. Most of these products increase the value to a range of 6° dKH to 8° dKH, which is ideal for the water. Select the appropriate product according to the amount of pond water. Each of the agents is sufficient for a certain amount of water in the pond, with the majority of the products being designed for a water volume of 2,000 to 3,000 liters.

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