Allelopathy is the process of inhibiting the growth of one species of plant by chemicals produced by another species.
Not much is know about this process with plants found in aquarium tanks. Most research is performed in the wild.
A good starting point on learning more on this process is to read the book:
ECOLOGY of the PLANTED AQUARIUM :A Practical Manual and Scientific Treatise for the Home Aquarist by Diana Walstad as it has a chapter on Allelopathy.
- See Walstad for where to obtain the book.
- See Allelopathy - chemical warfare between aquatic plants by Doctor Ole Pedersen