Hi,
I'm in the hogwash camp and that's because I've not seen any evidence of it. I can intentionally trigger some forms of algal bloom and then intentionally get rid of it, and none of that requires alleopathy. It simply requires a combination of the basics. So the whole talk about alleopathy is problematic because those having problems in their tank will rush to their favorite cause, like lemmings, instead of performing the proper steps to improve the health of their tank. Polluted water, for example contributes to the triggers of algal blooms, and at the same time reduces the level of Oxygen in the surrounding area. I would rather the focus to be on keeping your tank clean by extreme water changes. That way you remove any trace of potential alleopathic chemicals while minimizing the risk of hypoxia, so it effectively becomes a non issue. If you don't keep the tank clean then people can argue till the cows come home about what chemical caused the algae or killed the plant or fish.
The function of algae is to clean up the environment. It's not to make your life miserable. They really do not care about poisoning other plants just so that they can dominate the scene. That really isn't their programming. That's another reason people have trouble understanding that nutrients don't cause algae. The future of algae is guaranteed by the duality of their nature. They can live for thousands of years as spores. They really have no interest in the vegetative state per se because it's energy expensive. They can travel the globe and re-populate almost at will, just as spores. The deterioration of an environment, such as a poorly managed tank, gives them an opportunity to bloom, for the express purpose of producing more spores, which will live, almost forever. So algae could care less about alleopathy because they rule the planet (we only think we do) and their future is ensured by the fact that they are spores.
Having said all that, BGA and some types of flagellates are known to release deadly toxins. Red tide is a famous example, but these are bacteria and phytoplanktonic.
Also important to note is that generally, plants are not just sitting there looking pretty. They actively change their environment to suit their needs. The thousands of chemical reactions that they initiate could, in some way be considered alleopathic, but really there is little relevance in looking at it this way. Plants are the ones responsible for the very Oxygen we breathe, for example. They Oxygenate the soil and in so doing exclude certain types of aneaerobic microorganisms within their rhizosphere. Is that alleopathic? Maybe.
The 21% Oxygen that is part of the atmosphere was put there strictly by photosynthesizing plants and bacteria, and most of the work is done these days by the great forests of the world and as well by Diatomic algae. So you might want to consider that the next diatom bloom you get, and be thankful that they are so resilient and that you have so much trouble killing them. That's actually a good sign.
Cheers,
