I know they love nitrates as do many corals, so I don't think nitrates are a problem. I have a 50 gallon tank and have transferred all of the corals once and the xenias have always been in high flow areas. I am not sure what is going on with yours, maybe they will pull out of it, they just get weird sometimes. My pom pom xenias look like they are dying some days but then spring back to life. Same with my finger leather coral, it won't exhibit its florets for three days and then displays them for a month, then back to looking dead. They are funny animals.