It's not your alkalinity, I keep mine at around 12dkh.
How much flow do you have? I've found that zoas for the most part like medium to high flow.
Your lights may be too bright, lower the whites and blues by half and see how the corals respond, if no responce its either bacterial or pest problem.
If bacterial, a one day treatment of chemiclean on my tank helped me a lot. It's up to you if you want to try it. Your skimmer will go crazy for a week or two after treatment but it really cleaned my tank so it was worthwhile for me.
You can also dip each coral in furan 2 (google "zoapox furan 2" and click the zoaid website for instructions).
Good luck.
Every tank is different... If i were to run my tank at 12dkh i would probably burn up everything..LOL