Plants (and algae) produce chemicals that inhibit the growth of other plants and algae. These "allelopathic" chemicals may cause mysterious incompatibilities in the aquarium. Untraced allelopathic interactions may help illuminate what hobbyists noted generations ago, that Vallisneria and Sagittaria seldom thrive in the same water. The release of inhibiting chemicals is widespread in the plant kingdom. Among terrestrial gardeners, Black Walnut trees (Juglans regia) are notorious for the way they inhibit even grass from growing, wherever their roots spread and their leaves fall. Different plant species produce different allelochemicals. Terrestrial plants tend to rely on alkaloids (dare I mention tetrahydrocannabinol?), while aquatic plants more characteristically produce phenols. The actual chemical interactions are beyond an amateur's tracing, but I remove unidentified, perhaps allelopathic molecules by adding polyfilters in my filter units from time to time and keeping up with regular partial water changes. Diana Walstad devotes a full chapter to questions of plant allelopathy in her influential book Ecology of the Planted Aquarium, 1999.
