There are several potential reasons a octocoral like Xenia will fail.
This species is commonly used to export nitrates in some systems. They like nitrate. Scrub the water to SPS
standards and Xenia colony can fail, or slows down growth significantly.
It hates temperature spikes. It can set off a entire colony failure.
Some colonies seem to have a time limit built in. They get so big, then die back. This die back can be triggered by any number of events.
Lots of soft corals do this "cycle" thing as a result of sometimes tiny changes. One of the challenges in having SPS
and hard corals in the same system is this difference in nitrogen levels. JMO
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