Unless new research was done since I read up in the 2000s, phytoplankton does nothing for zoas - even if caught (and they will catch some), it costs them more energy to process than they get in return. Some zooplankton might be of benefit, but nobody is for sure. In either case, if the feedings raise the N and P levels even a bit, then they are hurting since the zoas are capable of feeding themselves 100%.
Don't think just because they grab stuff that it is helping them. The same is true for BTAs where smaller ones can expend more energy on feeding than they get out of it. Sometimes, just letting them be is better.
There was a study on reefkeeping mag, or the like, about 10 years ago that took some larger polyps and targed fed them pellets on one half of the rock and nothing on the other half. Neither grew better than the other. You can do this yourself with some tweezers and breaking a current colony in half.