My theory:
The metals contained within zeolites, including iron and aluminium provide one food source for metal eating bacteria, which create a biofilm around the zeolite, which in turn creates a micro environment for sulfur reducing bacteria (Desulfovibrio species) which produce (among other things) hydrogen sulfide. Which kills algae, and aparently some by products can interfere with the cadmium reduction method used for testing nitrate.
Zeofood i think also plays a big part in promoting the growth of these metal eating bacteria, by providing acetate which is readily used by them.
Of course, just a theory. Have fun researching that, there is some interesting stuff about those metal eating bacteria.
Layton