Carbon dioxide

What is carbon dioxide exactly and why is it important?

Carbon dioxide (CO2) is vital for the growth of aquatic plants. If there is enough CO2 available in free form for aquatic plants, they can grow well, converting organic matter using light and releasing oxygen. This is known as ‘assimilation’. Incidentally, CO2 levels fluctuate during the daily cycle. Underwater plants absorb CO2 using light and then release oxygen. As a result, CO2 levels fall during the day. At night, the process is reversed: the plants absorb oxygen and release CO2 (albeit to a limited extent)

Micro-organisms, the largest producers of CO2, are active throughout the daily cycle. As a result, CO2 will accumulate at night and levels will fall significantly later in the afternoon. This process only occurs in water bodies that function properly. In other words, when there are underwater plants growing and when the micro-organisms are active enough. We can monitor this process using the pH value. If the pH value is relatively low in the morning (pH 7-8) and relatively high in the evening (pH 8-9), the pond environment is functioning properly. The plants will grow well and the water will be clear. However, an unchanged pH (9 or higher) in the morning and evening indicates stagnation: the underwater plants are not growing and the pond is full of algae. The pH will fall after the growing season. The growth of aquatic plants (and algae) stagnates and so they stop absorbing CO2. The CO2 level in the pond will gradually increase. If the GH value (see GH chapter) is high enough, the excess CO2 can bond to form carbonate and the water will not acidify. The pH will not fall below pH 7. However, if the GH value is too low, there will not be enough calcium to bond the excess CO2. The water acidifies and there will be an oxygen deficiency. The pH value in this case will fall to below pH 6. You can use a simple Aqua test set to determine the pH very accurately

Green pond water

My pond water is green, what should I do?

If your pond water is green, then it is very likely that algae are the problem. You should first identify the type of algae in the pond. If green particles are floating in the water, the problem is floating algae. A short-term solution is to use Velda Crystal Clear. Crystal Clear is an environmentally friendly product that gets dirt and algae to settle on the bottom and makes the water clear.

The long-term solution to this problem involves several steps. Apply a layer of substrate if there is none present. At the same time, add Bacterial (nitrifying bacteria cultures). Put more pond plants into the water. At this stage, floating plants such as nymphoides peltata, frogbit, Azolla, duckweed or water lilies are a good choice. Make sure the general hardness (GH value) is high enough. This should be at least 8°DH. If the value is lower, use Velda GH-Plus to raise it.

If you take all these steps, the environment will stabilise after a while and the floating algae will disappear. Once the water is clear, you can put in the oxygen plants. Good varieties are elodea, hornwort and pondweed. Another excellent way to improve the environment and make the water clear is to install the Cross-Flow Biofill. Velda guarantees a clear pond within three months with this product.


How long is my guarantee?

If a Velda product does not work properly, you can of course invoke our guarantee conditions. Velda guarantees that all its technical products will work properly for a period of two years after purchase.

How can I see whether I have a guarantee?

Go to our guarantee registration page on our website where we describe the steps you need to follow to see if you have a guarantee.

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