dwalstad

There's considerable information about allelopathy in scientific literature. My book contains an entire chapter devoted to allelopathy (pp. 33-52) with 90 plus scientific references.

Allelopathy is subtle and difficult to demonstrate experimentally. Nevertheless, several experimental studies have proven alleopathy where:

• a plant species inhibits another plant species via the water
• a plant species inhibits another plant species via the sediment
• a plant species inhibits an algae species (e.g., the alga-inhibiting chemical Tellimagrandin II was isolated from Myriophyllum spicatum)
• an alga species inhibits another alga species

You can't really think about allelopathy without thinking about chemical evolution and the plant's native habitat. A plant species develops (over eons of evolution) to protect itself against the other plant species and algae within its native habitat. It produces and releases specific chemicals designed to compete with the specific plants and algae in its environment. Plants are "master chemists" (realize here that they have no other protection). Individuals of a plant species that "design" (via a mutation) a better protective chemical are the ones that survive and propogate.

Allelopathy is chemical warfare developed over eons, and there are a lot of chemicals.

These chemicals may or may not work when you throw this plant species into an aquarium. Here, the plant species is confronted with a completely different assortment of plants an algae that may (or may not) be susceptible.

As to allelopathy and CO2. Allelochemicals are not essential to plant metabolism; they are strictly for defense. Therefore, if a plant is barely hanging on, it might not be able to produce allelochemicals. In contrast, if the plant is healthy and getting enough CO2, it will be better able to produce compounds that may (or may not) protect it.