Allelopathy in Planted Aquariums: What the Evidence Shows
Chemical inhibition between aquatic plants, algae and cyanobacteria: known allelochemicals, laboratory and field evidence, why it is hard to separate from nutrient competition, and aquarium relevance.
What allelopathy means
Allelopathy is the release by an organism of one or more biochemicals that affect the germination, growth, survival or reproduction of other organisms. The term was coined in 1937 by the Austrian botanist Hans Molisch for chemical interactions through which one plant inhibits its neighbours. In 1996 the International Allelopathy Society broadened the definition to secondary metabolites produced by plants, algae, bacteria and fungi. The effects can be detrimental (negative allelopathy) or beneficial (positive allelopathy) to the target organism (Wikipedia, Allelopathy).
According to a review by Elisabeth Gross in Critical Reviews in Plant Sciences (2003), allelopathy occurs in all aquatic habitats, marine and freshwater, and involves cyanobacteria, micro- and macroalgae and flowering plants. Inhibition of photosynthesis is the most frequent mode of action, and the strength of the interaction is modified by nutrient availability and by bacterial breakdown of the released compounds.
Peer-reviewed evidence from aquatic plants
- Myriophyllum spicatum (Eurasian watermilfoil) releases polyphenols that are active against algae and cyanobacteria. The main active compound, tellimagrandin II, inhibits extracellular algal enzymes and, according to Leu et al. (Plant Physiology, 2002), also blocks photosystem II at a site different from that targeted by common herbicides.
- Hilt and Gross (Basic and Applied Ecology, 2008) list Myriophyllum, Ceratophyllum, Elodea, Najas and certain charophytes as allelopathically active genera that are among the most frequent submerged plants of temperate shallow lakes. They propose allelopathy as one mechanism that may help stabilise clear-water states.
- The same review reports that phytoplankton sensitivity differs between and within diatoms, cyanobacteria and green algae, and that epiphytic algae growing on the plants appear less sensitive than free-floating phytoplankton.
- A GC-MS survey of 11 freshwater macrophytes, including Myriophyllum spicatum and Ceratophyllum demersum, detected 137 major low-molecular-weight compounds, with fatty acids forming the core fraction in every species (Kurashov et al., Molecules, 2026). The authors present these as candidates for further bioassay and field testing rather than as proven allelochemicals.
Why the evidence is hard to obtain
The central difficulty is separating allelopathy from resource competition. Allelopathy adds an inhibitory chemical to the environment, whereas competition removes something that is limiting, such as light or nutrients, and both can produce the same outcome (Wikipedia, Allelopathy). Experimental tools used to tell them apart have their own problems: activated carbon, added to adsorb allelochemicals, has been criticised because it can itself change nutrient availability. In terrestrial research, key papers attributing an invasive weed's success to the compound catechin were later retracted or substantially corrected after fabricated data were found.
In aquatic work, the most common approach has been to test plant extracts or purified compounds on algae. Hilt and Gross state that final evidence requires more realistic in situ experiments, which have been carried out only for selected species, and note that the relevance at ecosystem level is debated. The role of allelopathy in suppressing epiphytic algae, the group most often seen on aquarium plants, is less well understood than its effect on phytoplankton.
Recent studies show both sides. In indoor co-culture, Ceratophyllum demersum and Myriophyllum spicatum suppressed the cyanobacterium Microcystis aeruginosa mainly by rapidly taking up nitrogen and phosphorus, leading the authors to conclude that nutrient competition was probably the dominant mechanism (Tang et al., Microorganisms, 2025). Vallisneria natans was found to recruit potentially algicidal bacteria on its leaf surfaces when exposed to Microcystis, so part of the suppression attributed to the plant may come from its microbiome (Gao et al., Frontiers in Plant Science, 2025). In contrast, a two-season field study in a eutrophic lake found that phenolic content in the water was the strongest predictor of mat size of the filamentous alga Cladophora glomerata and the duckweed Lemna minor, consistent with an allelopathic advantage for the alga, but acting together with pH, oxygen and dissolved solids (Ecology and Evolution, 2026).
Limits in a closed aquarium
Most of this research comes from lakes and laboratory cultures. Plant density, water volume per plant, nutrient loading and microbial communities in an aquarium differ from those systems, and the sources reviewed for this article contain no measurements of allelochemical concentrations in home aquariums. Microcosm experiments also show that results depend on ecosystem type: a mixture of macrophyte-derived acids reduced cyanobacterial biomass more strongly in microcosms containing fish than in simpler ones, because grazing and nutrient excretion also influenced the outcome (Kurbatova et al., Toxins, 2023).
Practical relevance
The documented aquarium practice relies on competition rather than chemistry. Nursery start-up guidance commonly suggests planting fast-growing auxiliary plants such as Egeria densa, Ceratophyllum demersum, Limnophila sessiliflora, Hygrophila difformis and Ludwigia repens in a new aquarium, on the reasoning that the extra plant mass absorbs nutrients from the water so that algae have less available, and removing them stepwise once the main plants are established. Several of these genera are also among those studied for allelopathic activity in lakes, so an aquarium with vigorous growth of such plants may involve both mechanisms, but the evidence reviewed here does not allow their contributions to be separated.