How to fertilize aquarium plants properly

While land plants take up their nutrients through their roots, aquatic plants can partly do so through their leaves. In principle, one can distinguish 3 types of nutrient uptake in aquarium plants:

Type 1. Nutrient absorption exclusively through the roots

Type 2. Nutrient absorption exclusively through leaves and stems

Type 3. Nutrient uptake through both the roots and the leaves and stems

Logically, type 1 plants need a nutrient-rich, well-fertilized substrate. Type 2 plants, on the other hand, should be used in an unfertilized substrate that is as low in nutrients as possible and made of sand and gravel without the addition of clay, while the aquarium water must contain a sufficient and balanced supply of nutrients.

Stem plants take up their nutrients through the leaves, the roots usually only serve to anchor them in the ground. Water goblets (Cryptocoryne) and sword plants (Echinodorus), on the other hand, have a highly developed, efficient root system with which they can extract the necessary nutrients from the surrounding soil.

Care effort and difficulty of care

Since different types of plants are usually cared for together in an aquarium, it makes sense to combine the liquid fertilization of the aquarium water with a nutrient medium from which the plants and their roots can extract the trace elements.

In addition, in most cases it is necessary to fertilize with carbon dioxide. Carbon is the most important element for the building metabolism for the plants and as a rule they take up this carbon in the form of the easily available carbon dioxide dissolved in the water.

  • The amount of maintenance that must be carried out for each type of aquarium plant depends primarily on two factors, the amount of light available and the need for carbon dioxide.
  • Plants that are easy to care for get by with little light and actually do not need any additional CO2 fertilization. However, if there are no or only a few fish in the tank, the supply of CO2 stimulates plant growth even with low lighting.
fertilize
An aquarium that has been properly fertilized

Carbon dioxide

The carbon dioxide comes from the air and the respiration of fish, microorganisms and plants into the water, where it is largely converted into carbonic acid.

Carbon dioxide and carbonic acid are immediately available as carbon sources for aquatic plants. If both are consumed, then the aquarium plants get the carbon from the lime dissolved in the water, characterized by the carbonate hardness of the water. This reduces the carbonate hardness, and this “biogenic decalcification” precipitates lime, which is deposited as a gray coating on the plant leaves. Above all, the lowering of the carbonate hardness makes the pH value unstable, which must be avoided as far as possible in the aquarium.

Aquarium Fertilizer Basics

Since the aquatic plants photosynthesize only in light, they only need carbon dioxide fertilization during the day. There are various technical options for CO2 fertilization.

The best solution is a controlled release of CO2, which is based on the daily needs of the plants. In addition, an endurance test uses different colors to indicate whether the water contains too little, the right amount or too much carbon dioxide.

Such plants are marked with simple or undemanding in the species description in the profile under care. Somewhat more demanding plants (care category: medium) need a medium to high lighting intensity.

An artificial supply of CO2 is not essential, but is recommended so that especially colorful plants develop in a correspondingly splendid and dense manner.

Category 3 includes the particularly demanding or difficult to care for plant species (care category: high, difficult or demanding): They usually need very intensive lighting of at least 1 watt per liter and a continuous supply of carbon dioxide.

The optimal range of CO2 concentration per liter, which can be different for each species, can be adhered to because too much CO2 also damages the plants.

Other important macronutrients

Other important macronutrients in terms of the required quantities are nitrogen (usually absorbed by plants as nitrate, sometimes also as ammonium), phosphorus (mainly orthophosphate dissolved in water) and potassium.

Nitrogen and phosphorus are usually present in the water through the decomposition products of fish food leftovers and the excrement of the fish and do not have to be replenished. If the aquarium is densely overgrown and has relatively few fish – this is usually the case, for example, with the so-called natural aquariums – then you should help with a special NPK fertilizer.

Among the minerals and trace elements, iron is very important, especially as a component of chlorophyll, but also in the red color of the plant leaves. If there is a lack of iron, the leaves will fade. However, too high a concentration of iron dissolved in the water can damage the plants.

When setting up the new aquarium, fertilizers are only added after a few weeks, when the plants have grown. However, carbon dioxide is added right from the start in order to promote growth and the color intensity of the plants.

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