Phosphates don't have an impact on pH, but alk
can lower Alk
if used in too high a quantity and impact ph.
I think you'd notice your nem not expanding if it weren't happy. As you can see, when they are happy and fed, they can really expand.
I don't need/want to feed my nem silversides anymore as it is already huge enough to host a pair of gold bar maroon clowns. Plus my clowns will bring scraps to it. It now gets enough from my daily squirt of frozen food mixture and pellets that are fed 4 times a day to the tank. Yes, it even eats pellets now. Caution, I feed heavy but my tank has become a medium/high demand tank. I also have ability to skim very heavy. Balance.