What kind of plankton?
Most stonys do the majoritiy of feeding at night.
I feed large food like zooplankton, Rods food, cyclops, etc. during the day so the fish can eat the leftovers (just puor some food flavored water in the tank and in 15-20min feeding tentacles come out). I feed phyto, ChromaMax, nauplii, and the occasional cyclops and rods in the morning before the lights come on.