coralline seem to be one of those things that can grow great in some systems, and do craptastic in very similar (and sometimes "better") systems. It's a crapshoot, and if it doesn't grow for you, then tough noodles unless you change something.
Ya might want to check Mg, as mentioned. I got my coralline going by scraping off some of the existing algae and dispersing the shavings around the rocks (pumps off for half an hour)
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