Corals such as zoas need good water parameter to thrive. Calcium, alk
, temp, light, some nitrate as well as phosphate. Right amount of Flow is essential, not necessarily for food but to absorb what's in the water such as trace elements. Without fish or feeding you will not get the phosphate and nitrate they need.
Edit: Any idea why you need the right amount of light? It is to process the food via photosynthesis. Just like nem, algae grows inside them. That's why you need those phosphate (less than .05 ppm
) for the algae.