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Corals “sulk” when they are stressed, uncomfortable, or trying to adjust. You might see them shrink, close up, look dull, or stop extending their polyps. This can be scary, especially if you are new to reef tanks. The good news is that most sulking has a clear cause and a clear fix. In this guide, I will show you how to recognize different types of coral sulking, find the root causes, and take safe, step-by-step actions to help your corals bounce back. The language is simple on purpose so beginners can follow along with confidence.
What “Sulking” Looks Like
Normal vs. Troubled Polyp Behavior
Polyp extension is a common sign of coral mood. In healthy conditions, many corals open up during the day to feed and breathe. Some corals, like many SPS (small-polyp stony corals), show small but consistent extension; LPS (large-polyp stony corals) inflate and sway; soft corals wave gently and look full. A sulking coral will stay tightly closed, pull in its tissue, or look shrunken for hours or even days. If your coral was open before and is now consistently tight for more than 24 to 48 hours, something likely changed in your tank that the coral does not like.
Color Clues: Browning, Paling, or Bleaching
Color changes also send messages. Browning often means the coral is producing more symbiotic algae (zooxanthellae), usually due to excess nutrients or not enough light. Paling can mean low nutrients, too much light, or a recent shock. Bleaching (bright white skeleton showing through or stark white tissue) is usually a sign of severe stress, often from light that is too strong or rapid swings in temperature, salinity, or alkalinity. If your coral is pale and closed, slow down, test your water, and make gentle changes.
Mucus, Shedding, and Slime
Some corals release mucus to protect themselves. Soft coral leathers often “shed” a thin waxy layer every few weeks. During this time, they can look miserable. Good flow helps them complete the shed, and they usually recover well. On the other hand, thick brown slime on LPS like hammers and torches can be a bad sign (often called brown jelly), which needs quick action. Learn what is normal for your specific coral and watch for unusual slime that smells bad or spreads to other corals.
The Big Three: Light, Flow, and Water Quality
Light Intensity and Acclimation
New corals often sulk when the light is too strong too soon. Even a coral used to high light can bleach if you jump straight to your normal schedule. Start new corals low and shaded, then slowly increase light over 1 to 2 weeks. As a simple guide: soft corals often like around 50 to 150 PAR, many LPS like 75 to 200 PAR, and many SPS like 200 to 350 PAR. If you do not have a PAR meter, watch the coral: bleaching or shrinking suggests too much light; browning and stretching towards the light suggests too little.
Flow That Corals Want
Flow brings oxygen and food and removes waste. Too little flow can cause sulking, algae settling on tissue, or even infections. Too much direct flow can cause corals to retract to protect themselves. Think “gentle sway” rather than “blast.” Soft corals and mushrooms like low to moderate, Euphyllia (hammers, torches, frogspawn) like gentle to moderate indirect flow, and SPS usually prefer strong, random, turbulent flow. If the coral is being whipped around or its tissue looks pulled, soften the flow. If detritus is settling on it or the coral stays slimy, increase flow slightly.
Stable, Clean Water
Water chemistry is the base your corals live on. Stability matters more than perfect numbers. Typical beginner-friendly targets: salinity 1.025 to 1.026 (35 ppt), temperature 24 to 26°C (76 to 79°F), alkalinity about 8 to 9 dKH, calcium 420 to 450 ppm, magnesium 1300 to 1400 ppm, pH 8.1 to 8.4, nitrate in the low single digits to teens (around 2 to 15 ppm depending on coral types), and phosphate around 0.02 to 0.08 ppm. Ammonia and nitrite should be zero. A reef with steady, reasonable numbers usually has happy corals.
Common Reasons Your Coral Is Sulking
New Tank or New Coral Stress
In a new tank, the microbiome is still young. Corals may sulk simply because the system is not yet mature. In an established tank, new corals often sulk during the first few days. Give them time. Place them lower, provide moderate flow, and avoid changes while they settle. Many soft corals and LPS relax after a week once they realize your tank is safe.
Parameter Swings, Not Just “Bad” Numbers
Corals dislike sudden changes. A fast shift in salinity, alkalinity, or temperature can cause a coral to close for days. Even large water changes can shock corals if new water parameters are far from the display. Make changes slowly and test often. Use an auto top-off to keep salinity stable, and pre-heat and aerate new saltwater before water changes.
Chemical Warfare and Toxins
Corals release chemicals to compete for space. Leathers and some soft corals are famous for this. In mixed reefs, activated carbon and regular water changes help. Toxins can also come from the room. Avoid air fresheners, bug sprays, and cleaning products near your tank. Check your equipment for rust and use reef-safe magnets and tools. If you suspect contamination, run fresh carbon and do a moderate water change.
Pests and Predators
Closed polyps can mean pests. Zoa-eating nudibranchs, flatworms, red bugs, acro-eating flatworms, vermetid snails casting webs, aiptasia stinging neighbors, and even some fish nipping can all cause sulking. Inspect at night with a small flashlight. Look underside and between branches. Coral dips help reveal and remove many pests. Quarantine new corals when possible.
Nutrient Starvation or Excess
Very low nitrate and phosphate can starve corals and their symbiotic algae, causing pale color and poor extension. Very high nutrients can brown corals and grow algae that irritates them. Aim for balanced nutrients. Do not try to push nitrate and phosphate to zero. Feed your fish modestly, and if corals look starved, consider targeted coral foods once or twice a week, but do not overfeed.
Quick Diagnosis Checklist
10-Minute Visual and Smell Check
Look for tissue recession (skeleton showing), unusual mucus, brown jelly, stings from neighbors, bite marks, algae on tissue, or vermetid snail webs. Smell the water near the coral area; a rotten smell suggests decay. If tissue is melting or brown jelly is present, isolate and act quickly.
Test and Compare With Targets
Measure salinity with a calibrated refractometer, temperature, alkalinity, calcium, magnesium, pH, nitrate, and phosphate. Write down the results and compare to your targets. If anything is far off, plan a slow correction. If parameters are fine, look harder at light, flow, pests, or chemical warfare.
Light and Flow Spot Test
Check for hot spots under LEDs where light might be too strong. If the coral is bleaching or always closed in bright light, shade it or lower intensity by 10 to 15 percent and observe for a few days. For flow, watch polyp movement. If the coral is getting blasted or covered with detritus, adjust your powerheads for a gentle, random pattern.
Safe Fixes You Can Do Today
Gentle Recovery Steps
Do a 10 to 15 percent water change with well-mixed, temperature- and salinity-matched saltwater. Add fresh activated carbon in a high-flow area to reduce toxins. Clean filter socks or floss and empty your skimmer cup. Siphon any detritus near the coral. These simple steps remove irritants without causing new stress.
Lighting Ramp and Placement Moves
If you suspect light stress, lower the coral to the sand bed or reduce light intensity slightly. Over the next week, increase light in small steps while watching for improvement. New corals should go through a light acclimation period. Moving a coral many times a day causes more stress; make a change, then wait and observe for a few days.
Flow Tuning Without Blasting
Rotate your powerheads so the coral gets indirect, pulsing flow. Avoid aiming a jet straight at fleshy tissue. If slime or debris collects, add a little more random flow. If tissue looks torn or polyps are pushed flat, reduce intensity. The goal is calm swaying and clean tissue surfaces.
Feeding Smart
Some corals perk up with gentle target feeding. LPS often enjoy small pieces of mysis or high-quality reef pellets, but do not stuff them. For filter feeders and soft corals, use fine particle foods or reef roids sparingly. Feed at night if the coral naturally opens then. Reduce fish feeding slightly if nutrients are high to keep balance.
When to Use Dips, Quarantine, and Tools
How to Dip Corals Safely
Prepare a dip using a reef-safe coral dip product in clean saltwater following the label. Dip the coral for the recommended time while gently swishing. Rinse the coral in a second container of clean saltwater before returning it to the tank. Never pour dip water into your display. Dips are great for pests on softies, zoas, many LPS, and some SPS, but fragile SPS need extra care with exposure time.
Brown Jelly, RTN, and STN
Brown jelly (common on Euphyllia) is fast and dangerous. Isolate the coral in a container of tank water. Siphon off the jelly, perform a coral dip, and consider fragging away healthy heads from infected ones. Increase flow around the coral in the display and run fresh carbon. For SPS that show rapid tissue necrosis (RTN) or slow tissue necrosis (STN), stabilize parameters, increase oxygen and flow, and frag healthy tips if recession spreads. Often, stability and time are the best medicine.
Using Carbon, UV, and Skimming
Activated carbon helps remove toxins from soft corals and chemical spills. UV sterilizers can lower free-floating pathogens and help with water clarity, which can indirectly help stressed corals. A properly tuned protein skimmer increases oxygen and removes dissolved organics. Use these tools to support recovery, not as a reason to skip basic maintenance.
Species Notes: What Each Coral “Tells” You
Softies: Leathers, Zoas, Mushrooms, and GSP
Leathers sulk when shedding; strong, random flow helps them shed and reopen. If a leather stays closed beyond a week without signs of shedding, check for chemical warfare and run carbon. Zoanthids close from pests (nudibranchs, spiders), stings, and low flow; inspect closely and dip if needed. Mushrooms hate strong light and high flow; place them low and shaded, and they usually inflate nicely. Green star polyps spread fast and prefer moderate light and flow; if closed, check for algae growing over their mat.
LPS: Euphyllia, Acans, Favia, and More
Euphyllia like gentle sway and moderate light. If tentacles are thin and retracted, reduce flow or light a bit. If they are too puffy and not moving, increase flow slightly. Acans and favia prefer moderate light and gentle flow. When hungry or stressed, they pull tissue tight to the skeleton. Target feeding small amounts can help, but watch nutrients. Avoid placing aggressive LPS too close to neighbors; their sweepers can sting at night.
SPS: Montipora, Acropora, Birdsnest
SPS are more sensitive to swings. Many sulk because of alkalinity or temperature changes. They like stable parameters, high oxygen, and strong, turbulent flow. Color shifts and poor polyp extension usually mean either too much light too fast or unstable water chemistry. When in doubt, stabilize first, then adjust light slowly.
Water Parameters Simplified
Targets and Safe Change Rates
Salinity: 1.025 to 1.026. Change no more than 0.001 per day. Use RODI water for top off. Calibrate refractometer with 35 ppt solution.
Temperature: 24 to 26°C (76 to 79°F). Avoid swings greater than 0.5°C (1°F) per day. Use a reliable heater and a controller if possible.
Alkalinity: around 8 to 9 dKH. Adjust no more than about 0.5 dKH per day. Keep dosing consistent and test often.
Calcium: 420 to 450 ppm. Magnesium: 1300 to 1400 ppm. Adjust slowly and dose only after testing.
Nitrate: 2 to 15 ppm depending on coral mix. Phosphate: 0.02 to 0.08 ppm. If nutrients are high, reduce feeding a little, increase export (skimming, refugium, water changes). If too low, feed a bit more or shorten refugium light cycle. Make small changes and wait a few days to retest.
Testing Schedule and Avoiding “Chasing Numbers”
Test salinity and temperature daily when you are new, then a few times a week once stable. Test alkalinity every few days at first, then weekly. Test calcium and magnesium weekly. Test nitrate and phosphate weekly. If a coral is sulking, test more often until it recovers. Do not chase tiny changes. Pick reasonable targets and keep them steady.
Placement and Aquascape Tips
Height, Shadows, and Neighbors
Light falls off quickly with depth and angle. Place corals according to their light needs: softies and many LPS lower or mid, SPS higher. Watch for shadows from rock overhangs or tall corals blocking light. Leave space between aggressive corals. At night, some LPS send out long sweeper tentacles. Give at least several inches of room to avoid chemical and physical fights.
Mounting and Stability
Wobbly frags stress corals. Use reef-safe glue or putty to secure them. If a coral keeps falling, it will sulk. Also, mount corals so their tissue is not scraping rock edges or getting sanded by high flow. A stable, comfortable perch helps them open and grow.
Preventing Sulks Long-Term
Routine Maintenance Habits
Do regular water changes with well-matched saltwater. Clean pumps and return lines so flow stays strong. Replace carbon monthly if you keep soft corals. Empty the skimmer cup and clean filter socks or floss regularly. Make one change at a time and keep notes in a simple log. Small, steady care beats big, sudden fixes.
New Additions Protocol
Dip and inspect every new coral. Quarantine if you can. Light-acclimate and flow-acclimate all new pieces. Place new corals lower first, then move up slowly. Feed normally and avoid overreacting. Give each new coral at least a week to adjust before making big changes.
Handling and Hands-Off Policy
Corals do not like constant handling. Keep your hands out of the tank unless needed. If you must handle, rinse your hands well in plain water and avoid soaps, lotions, or sprays. Do not clean glass with household products near the tank. Use reef-safe tools and wipe drips right away.
24-Hour Rescue Plan and 2-Week Stabilization
Day 1: Calm, Clean, and Clear
First, stay calm. Do a 10 to 15 percent water change with matched water. Add fresh carbon. Clean mechanical filtration and empty the skimmer. Check salinity, temperature, and alkalinity; correct gently if off. Reduce light intensity by about 10 percent if you suspect light stress and ensure comfortable, indirect flow. Inspect for pests and dip if needed. Isolate severe cases like brown jelly. Do not feed heavily during the first 24 hours unless the coral shows strong feeding response.
Week 1 to 2: Slow and Steady
Test alkalinity, nitrate, and phosphate every few days and aim for stable targets. Gradually ramp light back up if color and extension improve. Adjust flow slightly based on polyp behavior. Feed modestly once or twice a week if the coral is responding. If you use a refugium or nutrient filters, tune them for gentle changes. Avoid moving the coral more than once a week. Keep notes so you can see patterns.
Extra Troubleshooting Tips
Algae and Overgrowth
Algae touching coral tissue can cause sulking. Gently brush it away with a soft toothbrush during a water change, increase herbivore crew if needed, and adjust nutrients to reduce algae growth. Keep detritus from building up around the coral base.
Stray Voltage and Equipment Issues
Faulty pumps or heaters can stress corals. Use GFCI outlets and a reliable heater controller. A titanium grounding probe can help, but fix the faulty equipment rather than masking the problem. Regularly inspect magnets and metal parts for rust and replace if needed.
Recognizing “Normal” Sulks
Leathers that are shedding, zoas after a dip, or LPS after a big meal may close for a short time. Nighttime polyp retraction is also normal for many species. If the coral reopens later and looks normal, it was probably a harmless sulk. Learn each coral’s routine so you do not panic over normal cycles.
Beginner-Friendly Equipment Checklist
Simple Tools That Make a Big Difference
Use a reliable refractometer calibrated with 35 ppt solution, not freshwater. Have quality test kits for alkalinity, calcium, magnesium, nitrate, and phosphate. A heater with a controller or temperature alarm helps prevent swings. Use RODI water for top off and mixing salt. Powerheads with adjustable flow and a light with a timer make consistency easy. An ammonia alert badge is useful for new tanks or quarantine.
Putting It All Together: A Practical Workflow
Observe, Test, Adjust, Wait
When a coral sulks, follow this order. Observe the coral and surroundings for pests, stings, or slime. Test key parameters. Make one or two gentle changes (fresh carbon, small water change, slight light or flow tweak). Wait a day or two. If things improve, continue slowly. If the coral worsens, consider a dip, relocation, or more focused action like fragging an infected area. Patience plus steady conditions usually wins.
Conclusion
From Sulk to Smile
Coral sulking is your reef sending a message. It might be asking for less light, smoother flow, cleaner water, or simply more time to settle in. Use the signs your coral shows—polyp extension, color, slime, and tissue health—to guide your response. Focus on the big three: light, flow, and stable water quality. Make small, careful changes, and allow time for recovery. Dip new corals, run carbon in mixed reefs, feed thoughtfully, and keep a calm routine.
With this simple, step-by-step approach, most sulking corals will open up again and thrive. Do not give up at the first sign of trouble. Reefing rewards patience, observation, and gentle hands. Keep notes, learn each coral’s preferences, and enjoy the process of helping your reef come back to life.