I think the ammonia problem, when we are talking about algae, is that fish and plant waste generates a nearly constant small amount of ammonia in the tank. The concentration will likely be too low to measure, but down at the substrate it could fluctuate and hit high enough levels to trigger algae spores to start the growth cycle. Removing that ammonia by filtering it out with a canister filter, for example, wouldn't be effective, because of the time it would take for that ammonia to make it to the filter inlet, where it would be very dilute anyway. But, plants growing well right there on the spot where the ammonia is generated, are effective just by being so close to the source, and plants quickly absorb that ammonia. This is how I think it works, anyway.

And, this suggests another idea: if the substrate were zeolite sand, and if you used an UGF, would those tiny spikes of ammonia get quickly sucked down thru the zeolite, be absorbed by the zeolite and rendered harmless? An obvious flaw in this is that the substrate could only absorb a limited amount of ammonia, but wouldn't the plant roots be constantly removing the ammonia to "recharge" the zeolite sand? Just an idle thought I had the other day.