It could be that the healthiest plant in an enviornment where it has all it needs met in ferts and co2, can then devote a greater portion of its energy to allelochemical release, as Diana suggested. Allelochemicals released by the plant may give them them edge over algae in an ideal enviornment where as in an unideal enviornment the plant's ability to use bio-warfare may be greatly reduced. Of course all this is completly a wash if you do daily water changes I guess.
