what would cause xenia to wilt?

ANYTHING!! As mentioend above xenia is a very funny coral that is often misunderstood. The only thing known for sure is it usually either loves your tank or hates it. Many people can keep it and it explodes in growth and others can keep everything but xenia. Mine dose well under moderate light and moderate flow with my nitrates around 5 in my skimerless system. There have been long periods where it doesnt pulse much for a month or so and then it will pulse like crazy for a few months, the only thing I seem to notice is smaller frequent water changes and keeping my alk up seem to help the coral pulse and grow rapidly.

 

Glad to hear your nitrates are dropping! I can't help much with the xenia. As others have said, they are a funny coral. What I can say, is that, unlike other corals, they do not have a mouth and it is know that zooxanthellae pretty much just provides, sugars, not so much nutrients. So, most corals need some nutrients from feeding. As xenia does not feed obviously, it is believed that they must rely more on absorbing nutrients from the surrounding water. Therefore, changes in nutrients may have a negative effect. This is somewhat speculative though.

 

I had xenia and GSP doing fine in my tank. I had a bad outbreak of brown algae and they all seemed to wither and suffer until the outbreak was cleaned up. They are doing fine again. Not sure what changed in the water parameters to affect them so badly but it did.