You may want to check for pests at night with a red light and a magnifying glass. Alternatively, pull out the corals and give them a good dip in pest killer. Or both!

Sometimes just a particular frag or colony of zoas may not do well. How long have you had them? Are all of your zoas looking sad? What kind ? Pics?

I've had some success with dosing Iodine. What supplements are you adding, if any?