Allelopathy between plants in the freshwater aquarium

Plants do not communicate. Or at least, that is what people think. In reality, chemical communication between plant organisms is one of the most active fields of research in biology over the last thirty years, and what emerges is anything but simple: plants “talk” to each other continuously through chemical compounds, they modify the environment around them to favour their own expansion, they produce substances that inhibit competitors or attract useful organisms. They do it on land, in woods, in grasslands. They do it underwater too, in aquariums, in a way that we often do not notice but that concretely influences the health and growth of every plant in the tank.

This phenomenon is called allelopathy. In the freshwater aquarium hobby it is one of the most ignored, most misunderstood and most practically relevant subjects that exist. Ignored because it is not visible. Misunderstood because its effects are almost always attributed to nutrient deficiencies, lighting problems or fertilisation mistakes. Practically relevant because it explains dozens of situations that otherwise remain unanswered: that plant that does not grow even with perfect values, that moss that wastes away next to the hornwort, that vallisneria that blocks the whole eleocharis carpet within a radius of twenty centimetres.

This guide is written for those who want to really understand, not for those looking for a list of compatible and incompatible plants to consult like a manual. Because allelopathy in the aquarium does not work with lists: it works with an understanding of the mechanisms, with observation and with the ability to reason about the system as a whole.

What allelopathy is: biology before the myth

The term “allelopathy” was coined by the Austrian botanist Hans Molisch in 1937 to describe the biochemical effects, both stimulating and inhibiting, that one plant organism exerts on another through the production of chemical compounds released into the environment. The original definition included both the positive effects (stimulation of the growth of nearby species) and the negative ones (inhibition). In the common language of the aquarium hobby, the term is used almost exclusively for the inhibiting effects, but this restriction is scientifically imprecise.

Allelopathic compounds, technically called allelochemicals, belong to very different chemical classes: phenols and phenolic acids, terpenes, flavonoids, glucosinolates, alkaloids, fatty acids. There are no “universal” allelopathic compounds: every species produces its own specific chemical profile, with effects that vary according to the target species, the concentration, the pH, the temperature and the presence of other compounds in the water. This complexity is one of the reasons why allelopathy is difficult to study in the laboratory and even more difficult to interpret in a home aquarium.

Allelopathy in water: an environment different from soil

In a terrestrial environment, allelopathic compounds are released into the soil, where they are partly adsorbed by the clay particles, degraded by the bacterial and fungal microflora, diluted by rainwater. Their effect is attenuated by this natural filtration system. In water the situation is radically different: the compounds disperse directly into the liquid volume, they reach the nearby plants through the flow of the water, they interact with the substrate in a different way from soil. In a closed system such as an aquarium, this means that allelopathic compounds can build up over time, especially if the filter is not efficient at degrading or removing them.

Activated carbon adsorbs many dissolved organic compounds, including some allelochemicals. It is one of the reasons why tanks managed with activated carbon often show fewer allelopathy problems than tanks without it. But it is also one of the reasons why removing the activated carbon from a tank where it had always been present can bring out problems of coexistence between plants that had remained latent for months.

How allelochemicals act on plants

The mechanisms of action of allelochemicals on target plants are multiple and often overlap. Understanding them helps to interpret the symptoms observed in the tank.

Inhibition of photosynthesis

Some allelochemicals, in particular certain phenols and flavonoids, interfere with the electron transport chain in photosynthesis. The visible result is a reduction in growth, often accompanied by yellowing or discolouration of the leaves that does not respond to fertilisation with iron or micronutrients. It is one of the most confusing patterns: the plant shows symptoms that apparently come from a deficiency, but adding nutrients solves nothing because the problem is not the availability of nutrients but the ability to use them.

Inhibition of the roots

Certain compounds released by the root exudates of some species inhibit the root development of nearby plants: they reduce the elongation of the roots, alter the permeability of the root membranes, interfere with the absorption of water and mineral salts. In the aquarium, this effect shows up as a stagnation in the growth of plants close to the roots of the allelopathic one, with short, poorly branched, often brownish roots.

Alteration of the substrate microbiome

Some plants modify the composition of the microbial community of the substrate through their root exudates. They favour certain bacteria at the expense of others, they alter the localised pH of the substrate, they modify the availability of certain nutrients in the immediate vicinity of the roots. These effects have indirect consequences for nearby plants, which find themselves growing in a microenvironment that is chemically different from the one further away from the allelopathic plant.

Interference with hormonal signals

Some allelochemicals mimic or interfere with plant hormones: auxins, cytokinins, gibberellins. The effects can be paradoxical: at low concentrations, certain compounds stimulate growth (a hormetic effect); at higher concentrations, they inhibit it. This explains why some plants in small quantities do not create problems but become problematic when they grow to dominate the volume of the tank.

The plants most involved in the freshwater aquarium

The scientific literature on the allelopathy of freshwater aquatic plants is still relatively limited compared with terrestrial botany, but the observations accumulated by the aquarium community over the course of decades provide a fairly clear picture of the most active species. It has to be said honestly: many of these observations are anecdotal or based on studies in conditions different from those of the home aquarium hobby. Allelopathy is a real phenomenon but its intensity in a tank depends on too many factors to allow absolute statements.

Ceratophyllum demersum: the champion of aquatic allelopathy

Hornwort is probably the freshwater aquatic plant with the most documented allelopathic activity. Studies published in scientific journals (not aquarium forums) have shown that it releases phenolic compounds and other secondary metabolites capable of inhibiting the growth of algae, but also of some slow-growing plants. In the wild, hornwort is a competitively very aggressive species that colonises bodies of water forming dense beds that exclude other plant species.

In the aquarium, hornwort creates problems above all when it is present in large quantities and close to sensitive plants. The species most vulnerable to its substances are the slow-growing ones with delicate leaves: Cabomba, Elodea, certain Myriophyllum. Robust fast-growing plants generally coexist without problems. A small quantity of hornwort as a floating plant to filter nitrogen rarely creates significant allelopathic problems.

Vallisneria: the allelopathy of carpets

Vallisneria is one of the most used plants in the aquarium hobby and one of the least suspected of allelopathic problems. And yet, the documented cases of incompatibility between vallisneria and carpeting plants are numerous. Eleocharis parvula and Lilaeopsis brasiliensis seem to be particularly sensitive to the presence of vallisneria in the immediate vicinity: they slow down their growth, they produce leaves shorter than normal, they often show basal yellowing that does not respond to fertilisation.

The mechanism is not completely clarified. It is hypothesised that the root exudates of vallisneria alter the microenvironment of the substrate in its immediate vicinity, making it less favourable for carpeting plants that require very specific substrate conditions. Distance seems to be a critical factor: vallisneria positioned 15-20 centimetres from the carpet rarely causes problems; vallisneria with rhizomes growing directly into the carpet almost invariably creates growth blocks.

Egeria densa: fast and allelopathic

Egeria is another fast-growing species with documented allelopathic activity. It releases compounds that inhibit the growth of slower and more sensitive species, in particular some foreground plants such as Micranthemum and certain slower-growing Ludwigia. In tanks where egeria is present in large quantities and is not pruned regularly, it can dominate the volume of water not only through competition for light and nutrients but also chemically.

Myriophyllum spicatum: allelopathy towards algae

Myriophyllum spicatum, the aquatic milfoil in some of its varieties, has been the subject of specific scientific research on allelopathy. It has been shown that it releases compounds, in particular tellimagrandin II and other hydrolysable polyphenols, capable of inhibiting the growth of cyanobacteria and filamentous algae. In the wild this capacity allows it to colonise bodies of water by reducing algal competition.

In the aquarium, this characteristic is a double-edged blade: the same activity that limits algae can negatively influence the sensitive plants present in the same tank. The effect is generally less intense than with hornwort, but it has to be taken into consideration in tanks with delicate plants.

Floating plants: a particular case

Floating plants, in particular Lemna minor (duckweed), Pistia stratiotes (water lettuce) and Salvinia, deserve a separate mention. Floating plants release their allelochemicals directly into the water column through the roots that hang downwards, and the compounds disperse throughout the volume. In tanks where the floating cover is very dense (over 50-60% of the surface), the accumulation of allelochemicals can become significant, especially in tanks with poor filtration or with infrequent water changes.

Duckweed is particularly active: in large quantities, it can inhibit the growth of algae (which is desirable) but also of sensitive submerged plants (which is undesirable). Controlling the floating cover is therefore important not only for reasons of lighting but also for chemical reasons.

How to recognise a problem caused by allelopathy

Diagnosing allelopathy in the aquarium is difficult because its visual effects are often identical to those of other much more common problems. Before concluding that the problem is allelopathic, it is necessary to rule out the more frequent causes.

The typical picture that should raise the suspicion of allelopathy is this: one or more specific plants show stunted growth, yellowing or progressive decline, while the plants of other types in the same tank grow normally. The water values are normal: correct pH, adequate hardness, regular fertilisation, adequate lighting. The struggling plant responds little or not at all to the addition of nutrients. The decline is localised: the plant close to the suspected species shows the worst problems, the plants of the same species positioned far from the suspected allelopathic one grow better.

This pattern, especially if repeated with the same species in different tanks or at different times, is strongly suggestive of allelopathy. Not definitive: there are other causes that can produce similar pictures, such as competition for light in shaded areas or localised micro-imbalances of nutrients in the substrate. But it is a reasonable starting point for an investigation.

The distance test

One of the most practical ways to check a suspicion of allelopathy is to observe what happens when you change the distance between the suspected species and the struggling plant. If by physically repositioning the plants (or removing the suspected species) the struggling plant starts growing again within 2-4 weeks, the suspicion of allelopathy is considerably reinforced. If nothing changes, the causes are probably elsewhere.

This test is simple, it does not require any particular equipment and it provides concrete information. It is far more useful than any unverified theoretical speculation about the specific nature of the compounds involved.

How to manage allelopathy in the tank

The management of allelopathy in the freshwater aquarium does not require special measures: it requires good general management practices and a few precautions in the layout and in the choice of plants.

Designing the layout

The choice of species and their positioning is the first level of prevention. Plants with high documented allelopathic activity should be positioned far from sensitive species, with distances of at least 15-20 centimetres between the rhizomes or the roots. In small tanks (under 60 litres), where the distances are inevitably reduced, it is better to avoid altogether the combinations known to be problematic.

Positioning in the traditional layout naturally helps: tall plants such as hornwort and vallisneria at the back, medium plants in the middle, carpets in the foreground. This arrangement also reduces root contact between incompatible species. It is not an infallible solution, because allelopathic compounds spread through the water regardless of the physical arrangement, but it reduces the intensity of the localised root effects.

Filtration and water changes

An efficient biological filter with abundant filter medium degrades a part of the organic allelopathic compounds through heterotrophic bacterial activity. Regular water changes, at least weekly, dilute the accumulated compounds and reduce the overall allelopathic pressure in the system.

In tanks with plants known for high allelopathic activity, more frequent water changes of 20-25% a week can make a significant difference to the coexistence with sensitive plants. It is not the most elegant solution, but it is often the most effective and the least invasive for the system.

Activated carbon: a temporary ally

Activated carbon adsorbs many dissolved organic compounds, including a part of the allelochemicals. Using it cyclically, for example for one week every month, can reduce the levels of accumulated allelopathic compounds without permanently eliminating the fertilising micronutrients (which would only be removed in the case of continuous use). It must be replaced every 3-4 weeks because it loses its adsorption capacity over time and can release the previously adsorbed compounds.

It is a useful tool as a complement, not as a stand-alone solution. If the allelopathic problem is serious, activated carbon attenuates it but does not solve it: it is necessary to act on the source.

Regular pruning

Plants in senescence (leaves that die and decompose) release greater quantities of allelochemicals than plants in active growth. Regularly removing old, dead or decomposing leaves reduces the release of compounds into the water column. Pruning the dominant allelopathic species, when they grow excessively, reduces their biomass and therefore the total quantity of compounds produced.

Plant diversity as a buffer

A tank with many different species, well balanced in their growth rates, tends to be more resilient to allelopathic problems than a tank dominated by one or two very vigorous species. Diversity creates a system where the allelopathic compounds of each species are partly counterbalanced by those of the others, and where competition for light and nutrients is distributed among many players instead of being concentrated on a few dominant ones.

This does not mean filling the tank with random species. It means choosing a considered mix where the fast-growing species (which tend to be the most allelopathic) are present in controlled quantities and balanced by medium and slow species positioned in such a way as not to be in direct contact with the roots of the former.

Allelopathy and algae: a complex relationship

Allelopathy between aquatic plants and algae is one of the most interesting and practically relevant aspects of the phenomenon. Some plants release compounds that inhibit the growth of certain types of algae. This is not by chance: it is a selective evolutionary pressure. Plants that manage to limit the competition of algae chemically have an advantage in environments where algae could otherwise dominate them.

Myriophyllum spicatum is the most studied example: its polyphenols selectively inhibit cyanobacteria and filamentous algae under laboratory conditions. But Ceratophyllum demersum, Egeria densa and other species also show inhibitory activity towards certain types of algae.

The practical complication is that these same compounds can also inhibit the sensitive plants in the tank. There is no allelochemical that inhibits only algae and is perfectly harmless to all the other plants: the specificity is partial, not absolute. Anyone who introduces allelopathic plants hoping to use them as biological control of algae must accept that the same substances could also influence the plants they want to protect.

Scientific insight: what the literature says

Scientific research on the allelopathy of freshwater aquatic macrophytes has grown significantly from the 1990s onwards, partly thanks to the interest in the use of plants as a biocontrol tool in surface waters contaminated by toxic algae. Some conclusions of the literature are relevant for the aquarium hobby.

Studies on Ceratophyllum demersum have identified specific phenolic compounds, in particular gallic acid and ellagic acid, as the main agents responsible for algal inhibition. The production of these compounds increases in conditions of high light intensity and high phosphorus concentration, which explains why the allelopathic problems of hornwort tend to be more intense in very brightly lit tanks with generous fertilisation.

For Myriophyllum spicatum, tellimagrandin II (an ellagitannin) has been identified as the main allelopathic agent in studies conducted on green algae and cyanobacteria. Its production is influenced by temperature: it increases significantly above 20°C. In tropical aquariums at 26-28°C, the allelopathic activity of this species can therefore be more intense than in tanks at a lower temperature.

An important consideration that the literature stresses: the effects observed in the laboratory often require concentrations much higher than those plausible in a home aquarium with active filtration and regular water changes. This does not mean that allelopathy in the aquarium is irrelevant, but that its intensity in a well-managed system is probably much more attenuated than some popular texts suggest. The margin between “documentable effect” and “significant practical problem” is wide and depends greatly on the specific conditions of the system.

Plant combinations: what works and what creates problems

Instead of a table (which gives a false sense of certainty in a field where the variables are many), it is more useful to reason by principles.

The most problematic combinations of all are those between plants with high allelopathic activity and sensitive slow-growing plants in direct contact or very close together. Ceratophyllum demersum with Cabomba caroliniana or with mosses such as Vesicularia dubyana often creates problems. Vallisneria spiralis with Eleocharis parvula in root contact almost invariably creates growth blocks in the carpet.

The combinations generally free of problems are those between robust fast-growing species of similar vigour: Hygrophila polysperma with Ceratophyllum, Vallisneria with Echinodorus, Egeria with Sagittaria subulata. These species have different chemical profiles but similar resistance to the allelopathic effects of the others.

The combinations to be assessed case by case depend greatly on the relative density of the plants, on the filtration, on the frequency of the water changes and on the size of the tank. A Vallisneria in a 200-litre tank with good filtration and weekly changes creates far fewer problems for the same Eleocharis than in a 30-litre nanocube with minimal filtration.

Conclusion

Allelopathy between plants in the freshwater aquarium is a real phenomenon, scientifically documented, with concrete practical effects on certain combinations of species. It is not a forum myth, but neither is it the explanation for all the growth problems encountered in a tank. It is one of the many factors that contribute to the biological balance of a closed aquatic system.

Understanding it means ceasing to look for the cause of every problem only in the chemical values of the water or in the fertilisation, and starting to look also at the interactions between organisms. Plants are not passive decorative elements that react only to the availability of nutrients and light. They are biologically active organisms that interact with their environment in ways we are still fully coming to understand.

A well-designed aquarium is not just an aquarium with correct values. It is an aquarium where the chosen species work together instead of fighting each other, where the biological dynamics were thought through before the plants were bought. This awareness, applied with common sense and without obsession over details, makes the difference between a tank that works and one that always leaves some question unanswered.

Practical boxes

Box 1: How to suspect a problem caused by allelopathy

Consider allelopathy as a possible cause when all these elements occur together. One or more specific species decline while the others grow normally. The water values are normal and the fertilisation is adequate. The struggling plants do not respond to the addition of nutrients. The decline is more intense in the plants close to robust, fast-growing species. The same problem has already occurred with the same combinations of species previously. If just one of these points is missing, look for other causes first: a specific micronutrient deficiency, localised lighting problems, mechanical root disturbance.

Box 2: Combinations to avoid in small tanks

In tanks under 60 litres, where the distances are reduced and the volume of water is limited, some combinations are systematically problematic. Ceratophyllum demersum with Cabomba, Elodea or mosses in direct contact. Vallisneria with carpets of Eleocharis parvula or Lilaeopsis. Egeria densa with Micranthemum or slow-growing Ludwigia. Floating plants with a very dense cover (over 60%) with sensitive submerged plants. In larger tanks with good filtration and regular water changes, these same combinations can work with the due precautions of positioning.

Box 3: Practical management in tanks with allelopathic plants

If you want to keep plants with high allelopathic activity such as hornwort or vallisneria in tanks with sensitive plants, follow these measures. Prune the allelopathic species regularly, do not let them become dominant in volume. Maintain a minimum distance of 15-20 centimetres between the rhizomes of the allelopathic species and the sensitive plants. Use activated carbon for one week a month to reduce the accumulated compounds. Increase the frequency of the water changes to 25-30% twice a week in the phases when the allelopathic plants are in active growth. Monitor the sensitive plants weekly and act immediately if they show signs of decline.

Box 4: Low allelopathic risk layout

To minimise the risks, design the layout following these principles. Background: robust fast-growing species such as Hygrophila, Bacopa or Limnophila, which tolerate the allelochemicals of others well. Sides: Echinodorus or Cryptocoryne, species of medium resistance that do not show significant sensitivity to most documented allelopathies. Foreground: carpets of Eleocharis, Lilaeopsis or Hemianthus callitrichoides, positioned far from the rhizomes of the background species. Floating: limited to a maximum of 30-40% of the surface to reduce the accumulation of allelochemicals in the water column. Absent: Ceratophyllum in tanks with sensitive plants, Vallisneria in tanks with small carpets.

FAQ

What exactly is allelopathy in the aquarium?
It is the phenomenon whereby some aquatic plants release chemical compounds (allelochemicals) into the water that influence the growth of other plants. The effects can be inhibiting (blocked growth, yellowing, decline) or, more rarely, stimulating. In the freshwater aquarium hobby the phenomenon is real but often overrated as a cause of problems that have other more common origins.

How do I know whether my problem is allelopathic or a nutrient deficiency?
The most useful signal is the response to fertilisation: a deficiency problem improves with the addition of the missing nutrient; an allelopathic problem does not respond or responds little. The localisation is indicative too: if the decline concerns specifically the plants close to a robust species while the same species further away grow well, the allelopathic suspicion is well founded. Ruling out the more common causes first (deficiencies, lighting, CO₂) is always the first step.

Is hornwort really that problematic?
It has the most documented allelopathic activity among the common plants in the aquarium hobby. But how dangerous it is depends greatly on the quantity present, on the filtration of the tank and on the nearby plants. A small quantity of floating hornwort in a tank with good filtration and robust plants rarely causes visible problems. The same plant in large quantities close to mosses or Cabomba in a poorly filtered tank can create serious problems.

Is vallisneria incompatible with all carpets?
No, only with some sensitive species in direct root contact. Eleocharis parvula and Lilaeopsis brasiliensis seem to be the most vulnerable. Hemianthus callitrichoides and Glossostigma elatinoides seem to tolerate it better, especially if positioned at an adequate distance. Positioning counts as much as the choice of species.

Does activated carbon solve allelopathy problems?
It attenuates the allelopathic compounds dissolved in the water but does not eliminate them completely and does not solve the problem at its root. It is a useful tool as a complement to more direct interventions (repositioning the plants, pruning the allelopathic species, increasing the water changes). On its own, without correcting the causes, it leads to a temporary improvement that disappears as soon as the carbon is exhausted.

Are allelopathic plants good against algae?
Some species (in particular Myriophyllum spicatum and Ceratophyllum demersum) have documented inhibitory effects on certain algae. But the same compounds can also influence the sensitive plants in the tank. It is not a simple solution: introducing allelopathic plants to control algae means accepting the potential side effects on the other plants.

Does allelopathy also work between plants of the same genus?
Yes. Some species show intraspecific allelopathy: they inhibit even the growth of individuals of the same species when the density becomes excessive. It is one of the mechanisms of self-regulation of density in natural environments. In the aquarium it shows up as a stagnation of growth in overcrowded tanks with a single species.

Are mosses particularly sensitive to allelopathy?
Based on the observations of the aquarium community, mosses (Taxiphyllum, Vesicularia, Riccardia) seem to be among the plants most sensitive to allelochemicals, especially those of Ceratophyllum. They grow better in tanks with few or no dominant allelopathic plants, good filtration and frequent water changes.

Does temperature influence the intensity of allelopathy?
Yes. The production of allelochemicals increases with temperature in many species. In tropical tanks at 26-28°C, the allelopathic activity of species such as Myriophyllum spicatum can be more intense than in tanks at 20-22°C. Temperature is one of the many factors that modulate the intensity of the phenomenon.

Can I use a water test to measure allelopathic compounds?
No, there are no commercial test kits for allelopathic compounds in the aquarium. The diagnosis has to be made by exclusion and by observing the patterns. In the laboratory, mass spectrometry and chromatography techniques are used to identify the specific compounds, but they are not accessible to the home aquarist.

Can the plants on sale in shops carry allelochemicals that have already been produced?
Allelopathic compounds are released continuously by living plants and tend to dissipate rapidly in the absence of the plant. A plant bought and introduced into the tank brings its own biological activity, not compounds produced by the tank it came from. It is not a vector of “allelopathic contagion”.

Does allelopathy explain why some tanks always stay free of algae?
Partly. A tank with dense, healthy plants in active growth has fewer algae for many reasons: competition for light and nutrients, allelopathic action, a balanced microbial system. Attributing the absence of algae solely to allelopathy is a simplification. The allelopathic effect contributes but it is not the only mechanism.

How long does it take to see the effects of allelopathy in a tank?
It depends on the intensity of the phenomenon and on the sensitivity of the plants involved. In serious cases (hornwort in large quantities close to very sensitive plants), the first signals can appear in 2-4 weeks. In more attenuated cases, it can take months before the decline becomes visible and clearly attributable to this cause.

If I remove the allelopathic plant, does the struggling one recover?
Often yes, if the decline has not gone too far. A plant that has suffered significant but not irreversible allelopathic effects starts growing normally again when the source of the problem is removed and the accumulated compounds dissipate with the water changes. Recovery times vary from one to four weeks.

Do carnivorous plants have allelopathic activity?
They are not among the most studied species in this respect. Aquatic carnivorous plants such as Utricularia produce specific compounds for capturing prey, but their allelopathic effect on nearby plants is not documented in any significant way. They are not generally considered problematic from this point of view.

Is allelopathy more intense in small or large tanks?
Tendentially more intense in small tanks, where the volume of water is reduced and the dilution of the compounds is lower. In a 30-litre tank, a quantity of hornwort that would not cause problems in a 200-litre tank can create significant effects. The scale of the tank is a factor to consider in the choice of plants.

Can I prevent allelopathy completely?
Not completely, because practically all plants produce some form of allelopathic compound. Its intensity and impact can be reduced significantly through the careful choice of species, considered positioning in the layout, regular maintenance and frequent water changes. The objective is not the elimination of the phenomenon but its management in such a way that it does not create practical problems.

Do liquid fertilisers reduce the effects of allelopathy?
Not directly. Fertilisers compensate for nutrient deficiencies but do not counteract the allelopathic mechanisms. A well-nourished plant can resist mild allelopathic effects better thanks to a more active metabolism, but in the presence of intense allelopathy even a perfectly fertilised plant will show difficulties.

Glossary

Ellagic acid: phenolic compound produced by some aquatic plants such as Ceratophyllum demersum. It has shown inhibitory activity on certain types of algae under laboratory conditions. It is one of the best characterised allelochemicals in freshwater macrophytes.

Gallic acid: simple phenol produced by various terrestrial and aquatic plants. Identified as a component of the allelopathic activity of Ceratophyllum demersum. It has an inhibitory effect on the photosynthesis of cyanobacteria and green algae at sufficiently high concentrations.

Allelochemicals: chemical compounds produced by an organism (in this context, by an aquatic plant) that influence the growth, survival or reproduction of other organisms. They belong to very different chemical classes: phenols, terpenes, flavonoids, alkaloids.

Allelopathy: biological phenomenon whereby an organism produces chemical compounds that influence the growth or behaviour of other organisms in the same area. In the aquarium hobby it refers mainly to the inhibition of the growth of one plant by compounds produced by another.

Auxins: class of plant hormones that regulate the growth and development of plants, in particular cell elongation and root development. Some allelochemicals mimic or antagonise the action of auxins, altering the growth of the target plants.

Cytokinins: plant hormones that stimulate cell division and differentiation. Some allelochemicals interfere with cytokinin receptors, altering the growth rates of the exposed plants.

Root exudates: chemical compounds released by the roots of plants into the substrate or directly into the water. They are one of the main vectors of allelochemicals in aquatic plants, in particular for species with a developed root system.

Macrophytes: botanical term for macroscopic aquatic plants visible to the naked eye, distinct from microscopic phytoplankton. In the aquarium hobby it includes all the submerged, emergent and floating plants used in tanks.

Secondary metabolites: compounds produced by organisms that are not strictly necessary for primary metabolism (growth, reproduction, energy) but have specific ecological functions: defence against predators, attraction of pollinators, competition with other species. Allelochemicals are secondary metabolites.

Polyphenols: large class of chemical compounds with a complex phenolic structure. It includes phenolic acids, flavonoids, tannins. Many allelochemicals of aquatic macrophytes belong to this class. They generally have good solubility in water and spread easily through the water column.

Rhizome: horizontal underground stem that acts as a storage organ and as a means of vegetative propagation in many aquatic plants (Vallisneria, Echinodorus, Cryptocoryne). Rhizomes in direct contact are one of the main vectors of root allelopathy.

Senescence: process of progressive ageing of a plant organ (leaf, branch, root) that leads to its death. Plants in senescence typically release greater quantities of allelochemicals than plants in active growth, which makes the regular removal of dying plant material important.

Tellimagrandin II: hydrolysable ellagitannin identified as the main allelopathic agent of Myriophyllum spicatum. It has shown inhibitory activity on cyanobacteria and green algae in laboratory studies. Its production increases with temperature and light intensity.