start dosing calcium. coralline is a calcium-based algae that needs calcium to grow.
i am currently using kent's liquid calcium, which seems to work well for coralline growth. you may want to go with bionic's two part solution. i have heard only great things. also heard some good stuff about kent's kalkwasser mix in combo with liquid calcium. bionic seems more cost efficient, but im not at all sure about that. i personally am using liquid calcium in combo with superbufferdKh which seems to do well for my small systems, however alkalinity has a tendency to drop quickly in my 29g.
what else is in there? how are you filtering the system?
you should dose iodine if using a skimmer as the skimmer will remove iodine quickly. i also dose strontium because i have a deal of inverts that use it regularly.
you should dose calcium in some way and consider beginning a dosage of iodine and strontium in the near future
IMO.
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SAVE A REEF - GROW YOUR OWN!
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29g berlin reef w/ 4.9
wpg and tons of current, 10g sump/fuge, and 12" CPR hang-on refugium
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SUPPORT YOUR REEF - START A REFUGIUM