I know that most zoos seem to like "dirty water" with slightly raised levels of nitrates and phosphates, but I wouldn't recommend that if you have other corals in the tank.
You can feed them regularly, I feed mine cyclops all the time, and they love it. I also give them cut up bits of krill and brine shrimp, and that helps with the growth a lot.
good flow, being closer to the light source (if possible), feeding (as ab420 mentioned) all have aided in good growth of my zoas in the past.
i typically have my flow in sort of a swirl type pattern. powerheads in opposing corners of the tank and pointing in opposing directions.
Like this (top down view):
----->
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