You know that 20PPM, is the highest your nitrates sould be for a fish only tank. How is your lighting system, since my wrasse will never come out of thier hiding spot under atinic lighting since they think it's still night time, but when my daylights turn on. they are swimming everywhere. Also, I read that they are not that hardy and feed primarely on the following:
The Red Coris Wrasse feeds mostly on shelled mollusks in the wild including hermit crabs, urchins, crabs, and occasionally tunicates. Aquarium specimens should have meaty items, vitamin-enriched shrimp, and brine shrimp three times daily.
I usually get hard to feed fish to start eating either by live brine soaked in selcon, Cyclop-eeze or Frozen food soaked in Garlic. If any of these don't work, I don't know what to say since it has worked for me.