By now, the opinion has become established in various internet forums and on social networks that the nutrient potassium is responsible for algae growth in the aquarium. In the aquarium scene, you could almost speak of a potassium phobia. We clear up a few things here.

Potassium as a nutrient

Just like nitrogen and phosphorus, the element potassium is one of the three most important macronutrients for plants. The term macronutrient makes clear that plants need this kind of food in larger amounts. Potassium makes up a fairly high share of 2 to 5% of plant dry matter! Inside plant cells, this chemical element serves to regulate pH and osmotic activity. It is not built into the cell structures but fulfills its task as a free cation (K⁺). This makes the nutrient very mobile in the plant.

Unlike nitrogen and phosphorus, potassium is not formed in the aquarium as an end product of the breakdown of organic waste. In natural waters, this element is available practically without limit. In a planted aquarium, potassium should therefore definitely be supplied through suitable fertilizers. Aqua Rebell Macro Basic Potassium, for example, is a dedicated potassium fertilizer. Most complete iron fertilizers contain some potassium, and this element is also contained in high concentrations in many NPK macronutrient fertilizers or nitrogen fertilizers such as Aqua Rebell Macro Basic Nitrate. Many hardening salts also bring a certain potassium content with them. So there are many different ways in which this important nutrient element can be supplied to aquatic plants.

A potassium deficiency usually shows up as small black holes, especially in older leaves. These so-called necroses can still grow larger over time. Other plant species react with older leaves turning yellow. In this case the leaf veins are yellowish as well. With a potassium deficiency, the new growth of aquatic plants can also turn out too small.

Aquarium plants can store potassium well and take up the element in larger amounts than they acutely need at any given time. This way, aquatic plants build up a reserve for bad times. Potassium is taken up preferably through the leaves. That makes it sensible to bring this element into the aquarium via the water column using suitable liquid fertilizers.

Potassium and algae

We clearly deny the claim that potassium is supposed to trigger algae growth. The mere presence of potassium in the aquarium water does not produce thread algae, green algae or any other types of algae. While nitrate and phosphate used to be frowned upon in aquaristics, potassium has now been handed the short straw and is used as the sole culprit for algae outbreaks in the aquarium. Potassium is and remains an important macronutrient for plants, even if it is certainly not used up as quickly and to the same extent as nitrogen, and it is not solely responsible for algae showing up. After all, there are certainly aquariums and fertilization concepts (such as the Estimative Index or the ADA fertilization concept) with higher potassium concentrations that work well and in which hardly any algae growth occurs. Thinking about it the other way around: algae can also develop in planted aquariums with low potassium levels. A low potassium level is therefore no guarantee of an algae-free aquarium.

What is decisive for healthy plant growth and low algae occurrence is the right ratio of the various nutrients to one another. This applies not only to the potassium level but also to nitrogen, phosphate and other macronutrients, CO₂ and iron, as well as other micronutrient elements. If relative deficiencies or extreme excesses occur in this nutrient structure, these imbalances can trigger algae growth or cause growth to stop in aquatic plants. Most of the time, the two go hand in hand. It is therefore important to strike a healthy balance of all nutrients and to carry out regular maintenance such as fertilization and water changes in order to keep exactly these ratios as even as possible in the long run.

Target values for potassium

So how high should the potassium concentration in a planted aquarium be? Levels of 5 to 10 mg/l K are given as rough guideline values. As described above, however, the nutrient ratios among one another should not be underestimated. The ratio of potassium to nitrogen is especially interesting here. Thanks to the enormous storage capacity of plants, potassium can be set at a 1:1 ratio with nitrogen (in the form of nitrate) without any problem. Even if the potassium concentration exceeds that of nitrate, algae problems do not necessarily arise. Many aquariums also work with a higher share of potassium in the water. Nevertheless, the aquarist has the option of changing the ratio through the choice of fertilizer. If the element potassium accumulates too much from the macronutrient fertilizers used so far (for example Macro Basic NPK, Estimative Index, Macro Basic Nitrate) and cannot be kept in check by regular water changes, a low-potassium (Macro Special N) or even completely potassium-free (GH Boost N) liquid fertilizer is recommended for supplying nitrogen.

The calcium-magnesium-potassium ratio

In our Aquascaping Wiki article on the calcium-magnesium ratio, one section also deals with the interplay of the three elements potassium, calcium and magnesium. Since there is competition for nutrient uptake by aquatic plants among the many metal ions dissolved in the aquarium water, nutrient blockages can occur with extreme imbalances. Not all plant species react with stunted growth and deficiency symptoms, however. In fact, in aquarium practice, the majority of common aquatic plants do not need such a precise calcium-magnesium-potassium ratio at all. The "regular aquarist" can therefore rest assured and does not need to rack their brains over it. Only for a few very special plant species that are difficult to keep, such as Ludwigia inclinata var. verticillata 'Pantanal', is a coordinated ratio of Ca:Mg:K of 2:1:0.5 very beneficial for the plant's health.

New potassium content in the fertilizers Aqua Rebell Micro Basic Iron and Micro Special Flowgrow

At the request of many customers, and to decouple the macro element potassium from the supply of micronutrients, the Aqua Rebell complete iron fertilizers Micro Basic Iron and Micro Special Flowgrow have had a different nutrient composition with regard to the nutrient potassium since 2018. The concentration was reduced significantly in both fertilizers (for example, the potassium content of Micro Basic Iron was reduced from 1.0 to 0.06 mg/l K when adding 10 ml of the fertilizer to 100 l of aquarium water). If the potassium concentration in the aquarium water is not sufficient for healthy plant growth, it can now be controlled much more precisely through liquid fertilizers in the macronutrient range or through a pure potassium fertilizer. This makes fertilizing the nutrient potassium considerably more flexible and targeted.