Few things are more frustrating for a reefkeeper than watching healthy zoanthids suddenly close up, shrink, or refuse to open. Zoas are known for being hardy and adaptable, so when they shut down, it's usually a sign that something in your aquarium needs attention.

This guide breaks down the 12 most common reasons zoanthids stay closed — and exactly how to fix each one quickly.

1. Sudden Changes in Lighting

Zoanthids don’t like abrupt changes in PAR or spectrum.

Signs

• Polyps stay tight and refuse to open
• Colors look washed out
• Zoas “stretch” toward the light

Fix

• Lower PAR intensity by 10–20%
• Shorten photoperiod temporarily
• Re-acclimate zoas using a 1–2 week light ramp

2. Too Much Flow

Direct blasts from powerheads cause irritation.

Fix

• Redirect flow so it’s indirect and gentle
• Aim for a soft sway, not whipping movement

3. Not Enough Flow

Dead spots cause detritus to settle around the polyps.

Fix

• Slightly increase random or alternating flow
• Blow debris off with a turkey baster during water changes

4. Zoanthid Pests (Nudibranchs, Spiders, Sundial Snails)

These are among the most common — and dangerous — causes.

Look for

• Tiny white nudibranchs
• Spider-like creatures on stalks
• Spiral-patterned sundial snails

Fix

• Dip coral in CoralRX, Revive, or Lugol’s
• Inspect for eggs and manually remove
• Repeat dips every 3–5 days for 2 weeks

5. Water Parameter Instability

Zoanthids prefer stability over perfection.

Problems often caused by:

• Alkalinity swings
• Zero nitrates or phosphates
• Rapid temperature shifts

Ideal parameters

• Nitrate: 5–15 ppm
• Phosphate: 0.03–0.10 ppm
• Alk: 8–9 dKH
• Temp: 77–78.5°F

6. Aggressive Neighbors (Chemical Warfare)

Nearby LPS (like Torch, Hammer, Candy Cane) can sting or release allelopathic chemicals.

Fix

• Move zoas away from aggressive corals
• Increase carbon filtration
• Perform a 10–20% water change

7. Bacterial Infections ("Zoa Pox")

Small yellow/white dots on the stalks or polyps often indicate infection.

Fix

• Lugol’s iodine dip
• Increase flow
• Improve water quality
• Remove severely infected polyps

8. Algae Overgrowth

Hair algae, turf algae, or film algae can physically smother zoas.

Fix

• Manually remove algae
• Reduce lighting intensity
• Add herbivores (trochus, emerald crabs)
• Lower nutrients slightly

9. Vermetid Snails or Bristleworms

Irritating nearby creatures may force zoas to stay closed.

Fix

• Remove vermetids manually or crush tube
• Relocate the frag or re-mount it

10. Predatory Fish or Inverts

Some species nip zoas, including:

• Certain wrasses
• Large hermit crabs
• Angels (especially dwarf angels)

Fix

• Observe tank at feeding time
• Remove or relocate nippers

11. Stress From Shipping or Fragging

Freshly shipped zoas often stay closed for 24–72 hours.

Fix

• Give them time
• Keep lighting low
• Place in low-flow areas initially

12. Irritation From Sand or Detritus

Sand blown onto polyps can cause them to stay closed for days.

Fix

• Blow debris off with turkey baster
• Adjust flow to avoid sandstorms

🧪 How Long Should Zoanthids Stay Closed?

• After minor irritation: 1–2 days
• After shipping: 2–4 days
• After major stress or pests: up to 1 week
• If closed longer than 7 days → you have a problem that needs intervention

🌱 Prevent Future Zoanthid Issues

✔ Dip every new coral

✔ Keep water consistent

✔ Maintain nutrients

✔ Provide moderate light

✔ Ensure gentle, indirect flow

A stable tank is a happy zoa tank.

🛒 Shop Healthy, Pest-Free Zoanthids at ExtremeCorals

All zoanthids at ExtremeCorals are:

• Professionally inspected
• Pest-dipped
• Photos taken under real lighting
• Labeled with accurate WYSIWYG detail
• Shipped overnight with care

👉 Browse Zoanthids: Zoanthids For Sale