Allelopathy is a phenomenon of true 'chemical warfare' between plants. Some plant species actively secrete allelochemical compounds (like phenols or alkaloids) from their roots or leaves into the surrounding water to inhibit the germination, root development, and growth of nearby rival plants, thus securing light and nutrients for themselves. In the aquarium hobby, the presumed mutual toxicity between Cryptocoryne and Vallisneria, or stunted growth near large bushes of Microsorum pteropus, is well known.

A specific plant, which previously grew perfectly well, suddenly stops growing or loses vigor after the introduction of a new, large species in the immediate vicinity. Unlike nutritional deficiencies, water parameters are perfect and distant plants thrive undisturbed. Often, stunting of roots directed towards the 'enemy' plant is noticed.

Planting strongly territorial species too closely together. Although in modern aquariums with large filters and water changes, allelopathy is often overestimated or inhibited by dilution, in small low-tech aquariums without regular water changes, toxic metabolites accumulate, stunting the growth of weaker species.

In case of suspected allelopathy, immediately increase the frequency and volume of weekly water changes to dilute and remove the dissolved allelochemical compounds. The temporary addition of high-quality activated carbon in the filter will absorb the organic toxins secreted by the plants.

Create a layout where species known for their strong presence are not planted in direct contact with small or delicate carpeting species. Performing weekly 30-50% water changes virtually eliminates any allelopathy problems.