In an aquarium focused on aquatic plants, indicator plants can be a good help for assessing the nutrient situation. In this article, we explain what an indicator plant is all about.

What is an indicator plant?

In relation to a planted aquarium, an indicator plant is a plant that can show any nutrient deficiencies relatively quickly through corresponding deficiency symptoms. The indicator plant reacts to a change in the nutrient balance faster than all other plants in the aquarium. The aquarist can thus counteract quickly with adjusted fertilization before deficiency symptoms spread to the remaining aquatic plants. Indicator plants are therefore preferably fast-growing stem plants with a fast metabolism. As a rule, they are plant species that react somewhat sensitively to nutrient deficiencies. An iron deficiency in particular is easily recognized in an indicator plant, since it shows through light, bleached shoot tips.

The point of an indicator plant is to recognize nutrient deficiencies quickly "at a glance". For this, you should know your plant well and know how it reacts to various nutrient deficiencies. The list of the different deficiency symptoms in aquatic plants should therefore be familiar.

Location of the indicator plant

The indicator plant should be positioned at a spot that corresponds as closely as possible to the normal conditions in the aquarium. A shady corner without current is rather unsuitable for this. The spot should also be chosen so that you can observe the plant well. The indicator plants should therefore not be excessively covered by other aquarium plants or hardscape. Indicator plants should of course always be able to show any deficiencies. Nevertheless, these species also have to be trimmed back now and then. Here it is worth leaving at least one or a few stems standing so that they can continue to serve as an indicator. This lets you bridge the time until the trimmed plant parts have recovered.

Suitable species for indicator plants

In general, fast-growing species that can react sensitively to nutrient deficiencies are suitable as indicator plants in an aquarium. These can be, for example, stem plants, but also quickly growing ground cover plants. It is an advantage to choose species that have rather large leaves, so you can recognize the deficiencies visually. We list some examples.

Heteranthera zosterifolia

The stargrass is a light green, fast-growing stem plant. The elongated leaves are quite wide. Heteranthera zosterifolia is an excellent nutrient indicator. With a lack of nitrogen (NO₃), the leaves begin to turn transparent before they wilt. If the water lacks phosphorus (PO₄), the plant's light green turns into a darker shade. An iron deficiency is also often shown in the form of pale shoot tips.

Glossostigma elatinoides

Glossostigma elatinoides is a fast-growing ground cover plant with light green, ovate leaves. For a ground cover plant, these are comparatively large, so deficiency symptoms are easy to recognize. The plant reacts sensitively to iron deficiency (whitish leaves), but also to nitrogen and carbon deficiency (yellow leaves). In addition, you can identify a lack of light in this foreground plant when the shoots point clearly upward. Although this is still more often the case initially during the switch from emersed to submersed, if Glossostigma elatinoides still grows upright even after several trimmings, this is to be interpreted as a lack of light.

Sagittaria subulata

Sagittaria subulata is in itself a rather robust and undemanding plant. This grass-like aquatic plant can be used in many ways and has light green blades that are quite wide. You can recognize nutrient deficiencies well on them. This foreground plant responds quite clearly to iron deficiency in particular and reveals a lighter coloration. The dwarf sagittaria is also well suited to aquarium beginners and is therefore a good choice even for less experienced aquarists if you want to use an indicator plant in the foreground to midground.

Reddish plants

Stem plants that turn red in particular, such as Rotala rotundifolia "Gia Lai" / "H'Ra", can also be used as indicator plants. They can show a lack of macronutrients such as phosphate and especially nitrogen, above all through a significantly more intense red coloration. These are especially species that tend to turn more orange-reddish under normal conditions. An NO₃ deficiency is indicated by a strong red up to magenta. Conversely, a more brownish or even greener coloration is a typical indication of a nitrate surplus. You can learn more about reddish plants and their coloring here.

Mayaca fluviatilis is also a stem plant, which is especially suitable as an indicator plant for iron deficiency. Although this aquatic plant has very small leaves, a lack of Fe shows clearly in this plant through faded shoot tips. These are also very easy to recognize with the naked eye. In itself, Mayaca fluviatilis is a rather more demanding plant.

If you do not have any of the species mentioned above in your aquarium, this is not a problem. The species in the aquarium that grows fastest is then simply designated as the indicator plant. Nutrient deficiencies can then be recognized fastest on it.
By the way: slow-growing species such as anubias are not good indicator plants. Deficiency symptoms that show up in them can sometimes be based on nutrient deficits from the past that occurred a long time ago and may have been corrected already. Here you should indeed fall back on species that grow as fast as possible. These plants then also react very quickly to the correction of deficiencies of certain nutrients. Chlorosis due to iron deficiency, for example, can be corrected again within a short time (hours to a few days). Leaves damaged by necrosis (mostly triggered by a macronutrient deficiency) can unfortunately no longer be "repaired" this way. Here you have to wait for the indicator plant's new growth.