Why Coralline Algae Isn't Spreading in Your Reef Tank

Coralline is a chemistry indicator, not just decoration

Purple coralline algae spreading across rock and glass is one of the better visual signs that your calcium, alkalinity, and magnesium are in a healthy, stable range — it's calcifying just like your coral, and it's sensitive to the same imbalances.

The most common blockers

• Low or unstable alkalinity. Coralline calcifies using the same alk/calcium chemistry as stony coral — if alk is low or swinging, growth stalls.
• Phosphate too low. Counterintuitively, ultra-low phosphate (near zero) can limit coralline growth just as much as excess nutrients limit it through competing algae.
• Insufficient flow in the area — coralline spreads faster in moderate, consistent flow versus dead spots.
• New rock/sand that simply hasn't had time — coralline spreads gradually from existing colonies, it doesn't appear instantly even in perfect conditions.

Speeding it up

Adding a small frag plug or rock with established coralline from a healthy tank "seeds" the population faster than waiting for spores to arrive naturally. Keep alkalinity and calcium stable for several consecutive weeks — coralline responds to consistency more than to any single high reading.