1
Core tests
Confirm salinity, temperature, alkalinity, nitrate, and phosphate.
Corals react strongly to swings. Test before changing placement, lighting, or dosing.
The Hidden ReefAquariums · Fish · Coral · Ponds
Reef troubleshooting
Coral Not Opening
Closed polyps, shrinking tissue, pale color, or sudden irritation usually means something changed. Check stability first, then look at light, flow, pests, neighbors, and recent dosing or dips.
- One coral closed Local flow, pests, neighbor stings, or placement may be the issue.
- Many corals closed Treat water quality, salinity, temperature, or dosing swings as likely.
- Pale or shrinking Review light intensity, nutrients, alkalinity, and recent changes.
- Tissue damage Inspect for pests, fish nipping, coral warfare, or infection.
First response
Check stability before moving the coral.
Repeatedly moving or dipping a stressed coral can make the problem worse.
2
Observe
Look for local irritation.
Check for pests, sand on tissue, fish nipping, neighboring coral sweepers, and direct blast from a pump.
3
Stabilize
Make one correction at a time.
Adjust slowly unless livestock are in immediate danger. Stability is often the treatment.
Visual clues
Read the pattern before reacting.
The number of affected corals tells you whether to look locally or system-wide.
Closed but neighbors look fine
Think placement, flow, light, pests, fish nipping, or coral aggression before changing the whole tank.
Tank-wide irritation
Check salinity, alkalinity, temperature, pH trend, nutrients, and recent dosing or water changes.
Closed, shedding, or waxy
Some soft corals shed naturally, but poor flow, low nutrients, or chemical irritation can extend the closure.
Retracted flesh
Watch alkalinity swings, low magnesium, too much flow, stinging neighbors, and damage to inflated tissue.
Pale, browned, or no polyp extension
Review light intensity, nutrients, alkalinity stability, pests, and rapid parameter changes.
Tissue peeling or rapid loss
Bring photos and water results quickly. Fast tissue loss needs a more careful plan than generic coral food.
What to check next
Start with stability, then inspect the coral.
Use this before changing lights, moving the coral, or adding supplements.
Question
Likely direction
Next action
Did salinity, alkalinity, or temperature swing?
System stress is likely.
Stabilize slowly, verify test accuracy, and avoid chasing numbers with large doses.
Is only one coral affected?
Local irritation is more likely.
Inspect for pests, sand, flow blast, coral warfare, fish nipping, or recent placement changes.
Did lighting or flow recently change?
Acclimation stress is possible.
Reduce sudden intensity changes and reposition gradually rather than making repeated moves.
Were additives, carbon, GFO, medication, or dips used?
Chemical irritation or nutrient swing may be involved.
Stop stacking corrections, run appropriate filtration, and confirm compatibility before more dosing.
Common causes
What usually keeps coral closed?
Coral stress is usually about instability, irritation, or a mismatch between species and placement.
Alkalinity or salinity swings
Fast changes can close coral even when the final number looks acceptable.
Too much or too little movement
Direct blasting can damage tissue, while dead spots allow detritus and film to build up.
Sudden intensity changes
New lights, cleaned lenses, moved rockwork, or schedule changes can shock coral.
Nitrate or phosphate mismatch
Very low or very high nutrients can both irritate coral, especially after rapid media changes.
Hidden irritators
Flatworms, nudibranchs, vermetid snails, aiptasia, and fish nipping can keep one coral closed.
Dosing or media changes
Fresh carbon, GFO, medication, dips, and supplements can cause stress when changed aggressively.
Helpful supplies
What you may need.
Bring test results and photos before buying a fix for a coral problem.
Reef test kits
Alkalinity, calcium, magnesium, nitrate, phosphate, salinity, and temperature are the starting point.
Shop test kitsSalt mix and salinity tools
Stable salinity and reliable measurement matter before supplements or coral foods.
Shop saltwaterPumps and wave makers
Better random flow can help coral stay clean without blasting tissue.
View equipmentReef lighting and acclimation
Lighting changes should be gradual, especially for newly moved coral.
View lightingCarbon, media, and reactors
Useful when chemicals, coral warfare, or dissolved organics are suspected, but avoid drastic swings.
View reactorsDip containers and coral tools
Photos, inspection containers, coral dip, and careful observation help identify pests safely.
Shop maintenanceWhat to avoid
Do not move the coral repeatedly, dose several supplements at once, or dip every coral before checking water stability. If tissue is peeling, pests are visible, or multiple corals close at once, bring photos and test results for help.
Compare with the algae guideOpen the quick-reference coral chart
