Allelopathy hasn't yet been demonstrated to be a characteristic of the aquatic plants we grow. Any such allelopathic chemicals would be organic, and therefore could be removed with activated charcoal in the filter. So, if a healthy tank were to have a load of AC added to the filter, that should cause an algae bloom, if allelopathy is involved in the lack of algae. As far as I know, no one has been able to do that experiment successfully.
It's magic! I can live with that.