I run zero nitrates on any test kit and xenia grows like a weed.
You can find xenia posts all over the place. Some tanks it will not grow. I had such a tank in the 2000's...
Your best bet, trust me, just find a reasonable place to be with parameters, alk, etc, feed the fish well, and keep things stable. It will grow. It melts when changes occur. ... just saying...
If parameters are stable for a few months and you still cant keep it from melting. Just stop trying for a year.... try again then. It can be the most finicky or the most indestructible coral, i swear. But if its not growing and your parameters are stable, just take a break from it and try again later... its a "$5 barely know you" frag from anyone who has it locally... get a long polyp gonni to hold you over