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.
Still, other experienced aquarists discount
these effects of allelopathic chemistry,
which would have to become diluted in natural
waters. They tend to credit incompatibility
among water plants to competitive nutrient
scavenging instead. See more about allelopathic
chemistry in the context of controlling algae here atthe site, and look at a 1997 post on this subject by Steve Pushak, archived
at theKrib.com.
One thought about "competition"
for nutrients: real competition, in which
the "loser" is notably repressed,
only obtains when the nutrient has become
so dilute it is effectively no longer available.
Until that stage is reached, "competition"
is merely a scramble among plenty.
