Very often, algae or growth problems in a planted aquarium arise not only from missing nutrients; often a mismatch between the aquarium lighting and the aquatic plants present is also the cause. We show you what to watch out for here.

The world of planted aquariums is very diverse. There are high-tech planted aquariums with many fast-growing stem plants, aquascapes with lots of hardscape, community aquariums with a few aquatic plants, or low-tech aquariums without CO₂ and only a few aquarium plants. All of these aquariums have their raison d'être, and there is no good or bad. What counts is that the ecosystem in the aquarium is in balance. With regard to the lighting, a plant selection matched to the respective brightness is of particular importance. It is therefore clear that demanding stem plants do not grow well in a low-light aquarium. A high-light tank that contains only a few mosses and anubias will surely tend toward algae. It is therefore a matter of choosing aquatic plants that suit the lighting, or, conversely, an aquarium lamp that is well proportioned to the aquarium plants.

Light categories

First, the brightness of plants has to be sorted into different categories. Here there are the gradations low, medium and high. Since aquarium lighting today is generally based on LED technology, the lumen rating of the lamp is decisive for classification. The luminous flux in lumens is usually stated in the technical data for LEDs. In the aquarium hobby, the lumen figure is then put in relation to the aquarium volume. This gives the so-called "lumens per liter" value (lm/l). Around 15 lm/l is regarded as the low-light range. 30 lm/l represents moderate lighting, and from 45 to 60 lumens per liter you can assume high light. Here are the categories once more in an overview:

• low light requirement: approx. 0.2 to 0.3 W/l with old light sources corresponds to approx. 15 to 20 lm/l with LEDs.
• medium light requirement: approx. 0.5 W/l with old light sources corresponds to approx. 20 to 30 lm/l with LEDs.
• high light requirement: approx. 1 W/l with old light sources corresponds to approx. 45 to 60 lm/l with LEDs.

You can find more info on this topic, by the way, here.

Plant categories

Aquatic plants can also be sorted into categories in terms of their demands. Many manufacturers (for example Tropica, Dennerle) already do this with a traffic-light system and label their aquarium plants with corresponding green, yellow and red labels. A green label stands for easy plants, yellow for moderate and red for demanding plants. We summarize this once more briefly in the following overview:

• green category - easy difficulty level: plants that are simple to keep with low demands.
• yellow category - medium difficulty level: aquatic plants with moderate demands.
• red category - high difficulty level: aquarium plants with high demands. 

In our online shop, too, you can sort aquatic plants by difficulty, in a range from very easy to very difficult. So here the categories are given in more differentiated form than the information from the nurseries.

Balancing light and plants

In principle, you can now relate the three gradations of plants (green, yellow, red) to the three ranges of lumens-per-liter values. Plants of the green category fit quite well into the low-light range. This includes most of the so-called shade plants, such as ferns, mosses, anubias, Bucephalandra, cryptocorynes and some Echinodorus.

Plants of the yellow category cope well with moderate light. Examples are not-quite-so-demanding stem plant species such as Rotala rotundifolia variants or Hygrophila species, but also many ground cover plants such as Montecarlo-3 or other foreground plants such as Pogostemon helferi. Most of the species available in the trade fall into this category and cope very well with the equivalent 20 to 30 lumens per liter.

And aquatic plants of the red category generally also need strong light. Often these are in fact reddish plants that only develop their reddish color from a certain brightness (between 30 and a maximum of 60 lumens per liter). Pogostemon stellatus may be mentioned as an example. Under lower light, these plants otherwise stay rather brownish or orange. Or the demanding plants then grow only very slowly and show a lack of light through long internode spacing. Very fast-growing and light-hungry ground cover plants also belong here, such as the popular Cuba pearlweed or Glossostigma elatinoides. As a rule, this includes fast-growing plant species with a high metabolism that is coupled to a lot of light.

Plant mass

But the sheer mass of plants is also decisive for a good balance. The stronger the light, the more (fast-growing) species should be set against it. Here, planting at least 75% of the area in the aquarium is recommended. For the moderate category at least 50% is required; for low light, 33% of the area is sometimes already enough.

It should also be mentioned here that demanding plants often do not cope well with weak light. They then often grow only very slowly and show light deficiencies in the form of long internode spacing. On the other hand, shade plants such as anubias do not feel particularly comfortable under high light either. Under increased brightness, these plants tend to get algae faster and more often get green spot algae growth, or red algae form on the leaf edges.

Plant colors / visual classification

A good aid for classification at a glance into the aquarium is to determine which shade of the aquatic plants predominates. Most low-light plants such as ferns and mosses have a dark green shade, which consequently fits well with the approx. 15 lumens per liter. Light green, brown-red and orange plants generally cope with 20 to 30 lumens per liter. Very strongly reddish plants, also in part with pink shades, tend to prefer 45 to 60 lumens per liter.

If a certain shade dominates across all the aquatic plants in an aquarium, you can use this as a rough guide to choose the right lighting for it. Of course, the plant mass should be taken into account as well.

Summary

Accordingly, the most important factors for a good balance in a planted aquarium can be summarized as follows:

• green category - easy difficulty level: plants that are simple to keep with low demands. Approx. 15 lumens per liter. At least 33% of the area planted. Predominantly dark green shades in the plants.
• yellow category - medium difficulty level: aquatic plants with moderate demands. Approx. 20 to 30 lumens per liter. At least 50% of the area planted. Predominantly light green, brown-red and orange shades in the plants.
• red category - high difficulty level: aquarium plants with high demands. Approx. 45 to 60 lumens per liter. At least 75% of the area planted. Predominantly light green, red, pink and orange shades in the plants.

Of course, aquatic plants, no matter what their demands, should also be fed accordingly well. In the easy category, a very sparse micro- and macronutrient fertilization is sometimes enough. Missing CO₂ is tolerated quite well by very easy plants, but it is beneficial for these varieties too. In the medium and difficult categories, CO₂ fertilization should be present, along with a balanced addition of micro- and macronutrients. In the red category, consumption is often also higher, so particular care should be taken here that the plants do not run into a nutrient deficiency. This applies above all to nitrogen and carbon.