I would respectfully disagree a bit here
I agree that the commonly expressed wisdom is that Xenia (and most other soft corals) like "dirty" tanks but IME how "clean" the water is doesn't seem to be a factor...
This is my Silver Tip Colony
FYI the rock it is on is almost a foot in length and 6 - 8 inches wide
This colony started as a single "stalk" with maybe 12 - 15 polyps on it that I got for $5 probably 1.5 years ago... it has also been "thinned out" at least threefold - so what remains in the pictures is probably just 1/3 of the colony I have grown.
As far as the conditions it is in:
Fairly "low nutrient" water. Not as low as an all SPS setup, but I run fairly agressive skimming, a macroalgae refugium for nutrient export, all "zeroes" for nO3 and pO4, and I run carbon and pO4 remover as well. Skimming, carbon, pO4 filtration 24/7/365 with regular changes of the media. I do not have an issues with cyano or hair / nuisance algaes.
Lighting - 250w MH 14K
Flow - medium; definitely to the point where the colony will "sway in the current" if that is helpful. I have noticed that whenever I shut off my pumps for more than just a few minutes the Xenia "pumping" lessens considerably.
Feeding - no "target feeding" of any corals
Supplementation - no trace elements or other water supplements used
Personally , I believe Xenia "likes" more lighting than most people believe (my colony is slowly working its way 'upward' in the tank - not 'downward', even under the halides); I also believe it likes a good amount of flow, semi-direct; as far as my "success" I have noticed better growth overall when I went to more frequent WCs (smaller volume) than in the past when I would do larger volume changes less frequently... I attribute that to the Xenia needing / liking some item out of the water (Iodine, Boron, Strontium, who knows?) that it gets from additions of newly mixed SW
I also have a good sized Pom Pom Xenia colony that I have already fragged several times and it still grows quit well - though not as fast as the Silver Tip does...