Tap water works fine to grow phytoplankton. The algae needs those items to grow. We want it to grow.
However, in our tank, we don't want algae to grow, right? So we use RO/DI water for the reef tank.
I'm just a hobbyist, so I try to reason out some of this stuff, and make assumptions based on gut instinct sometimes rather than based on scientific proof. My gut (and it could be wrong, right? ) tells me that the phyto uses up the stuff in tap water, and we dose phyto in small amounts. That is why I've never worried about it.