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Corals do not have faces, but they can look grumpy. When hobbyists say a coral is “sulking,” they mean the coral is closed up, deflated, withdrawn, or not showing its usual color or polyp extension. Sulking is a coral’s way of saying, “something is wrong.” The good news is that corals usually tell us what they need through their behavior. In this guide, you will learn why corals sulk, how water quality and stress play a role, and the simple steps you can take to help them bounce back.
What “Sulk” Looks Like and Why It Matters
Sulking is a temporary state where corals pull in polyps, shrink, or produce excess slime. Soft corals may droop or close up. LPS corals can deflate their fleshy tissue, exposing sharp skeletons. SPS corals often show poor polyp extension and a dull look. If sulking is brief and clearly linked to a recent change, it may be normal. Long or repeated sulking is a warning of stress or damage.
Spotting early signs helps you act before tissue recession, bleaching, brown jelly disease, or rapid tissue loss. Watch for tissue pulling away from skeleton (bad), a gray or brown film (infection risk), and sudden color fade (light or temperature shock). Quick response prevents small issues from becoming coral loss.
Fast Triage: What To Check First
Immediate checks in the first 10 minutes
Look for obvious problems. Is a powerhead off, heater light blinking, or skimmer overflowing? Did a fish start nipping a new coral? Is a coral stinging a neighbor with sweeper tentacles? If you see a clear cause, fix it right away and give the coral time to recover.
Check temperature and salinity first. These two cause many sudden sulking episodes. Make sure no pump is blasting tissue. If so, reduce flow or reposition.
Quick water tests in the first hour
Test alkalinity, pH, ammonia, nitrate, and phosphate. Alkalinity swings and low oxygen are classic causes. If ammonia or nitrite is above zero, this is urgent. Add extra aeration, reduce feeding, and perform a water change.
If you recently changed lights or moved the coral, suspect light shock. Dim LEDs, shorten the photoperiod, or use the acclimation mode if available.
Water Quality Basics That Make or Break Coral Mood
Temperature
Aim for 25 to 26°C (77 to 79°F). Keep daily swings small, within 0.5°C (1°F) if possible. Heaters and chillers should be sized correctly. Sudden heat or cold makes corals sulk fast.
Salinity
Target 35 ppt or specific gravity 1.025 to 1.026. Changes should be slow, no more than 0.001 per day. Top off with fresh RO/DI water only. Use a calibrated refractometer or high-quality digital salinity meter.
pH and alkalinity
pH naturally cycles. Normal is 7.9 to 8.3, with 8.1 to 8.3 ideal. Alkalinity supports stable pH and coral skeleton building. Keep alkalinity around 7.5 to 9 dKH for a mixed reef. Quick alk changes often cause sulking, especially in SPS. Avoid raising or lowering more than 1 dKH per day.
Calcium and magnesium
Calcium around 400 to 450 ppm and magnesium 1280 to 1350 ppm keep ions balanced. When magnesium is low, alkalinity becomes unstable and corals show stress. Dose only after testing and adjust slowly.
Nutrients: nitrate and phosphate
Corals need some nutrients. Ultra-low nutrients can starve them and bleach tissue; very high nutrients can brown corals and fuel algae. Good targets for a mixed reef are nitrate 2 to 10 ppm and phosphate 0.03 to 0.08 ppm. For SPS-focused tanks, many keep nitrate 5 to 10 ppm and phosphate 0.02 to 0.05 ppm. Zero is not the goal.
Ammonia and nitrite
Both should always be zero. Detectable ammonia means something major changed: a die-off, overfeeding, or a new uncycled filter. Immediate water changes and extra aeration help. Do not add new corals until it is stable.
Stability Over Perfection
Many beginners chase perfect numbers and end up with big swings. Corals prefer steady “good” over unstable “perfect.” Pick reasonable targets and hold them. Dose only what your tank consumes. Test regularly and adjust in small steps. Large water changes can be very helpful, but match temperature, salinity, and alkalinity to avoid shocking the system.
Think of your tank as a daily rhythm. Try to dose alk and calcium at the same times each day, and keep lighting and feeding schedules consistent. Regular habits create predictable chemistry and happier corals.
Light: Too Bright, Too Dim, Too Fast
PAR targets and signs of trouble
Soft corals often do well at 50 to 150 PAR. Many LPS are happy at 75 to 200 PAR. SPS usually want 200 to 350 PAR, sometimes more. If corals are stretching upward or losing color slowly, they may need more light. If they bleach quickly or remain tightly closed at midday, it may be too much light.
Acclimation matters
When you upgrade lights or bring home a new coral, reduce intensity and ramp up slowly over 2 to 3 weeks. Most modern lights have an acclimation mode. If not, shorten the photoperiod and add diffusers or raise the fixture. Screen layers can help during transitions.
Spectrum and photoperiod
Corals use blue light efficiently. Heavy blue is fine, but do not make sudden spectrum changes. A daily schedule of 8 to 10 hours of main lighting with short ramps at the start and end works well. Avoid large day-to-day changes in programs.
Flow: Gentle Sway to Strong Surge
Right flow for different corals
Soft corals enjoy gentle to moderate, random flow that makes them sway. LPS need enough to move tentacles but not so strong that flesh rubs on skeleton. SPS generally like strong, varied flow that prevents detritus settling.
Turnover guidelines and visual cues
Total flow of 10 to 20 times tank volume per hour suits softy tanks, 20 to 30 times for many LPS mixed reefs, and 30 to 50 times (or more) for SPS-dominant systems. Use visual cues: tissue peeling or constant retraction suggests too much; film algae or detritus collecting on the coral suggests too little.
Placement and pump patterns
Avoid direct, constant jets. Use alternating or random modes. Place corals so flow breaks around them and changes direction. If a coral is sulking, try a small move to an area with a different flow character rather than simply turning pumps up or down.
Chemical Stress: Toxins and Warfare
Allelopathy and coral wars
Corals release chemicals to compete. Large leather corals and some softies can irritate LPS and SPS, causing sulking and poor growth. Use activated carbon and regular water changes to reduce these compounds. Allow space between species and watch for long sweeper tentacles from LPS, especially at night.
Contaminants and metals
Rusting magnets, tools, or corroded pump parts can release iron or other metals. Household aerosols, cleaners, and smoke also harm corals. If you suspect contamination, run fresh activated carbon, add a Poly-Filter pad, perform a 20 to 30 percent water change, and stop using scented products near the tank. If available, ICP testing can identify metals and guide a cleanup plan.
Chemical filtration and skimming
Activated carbon is your friend. Replace it every 2 to 4 weeks or when you notice water yellowing. Protein skimmers help remove organics and increase oxygen. If you turn the skimmer off to feed, remember to turn it back on. Extra aeration during emergencies can prevent losses.
Nutrients and Feeding: Not Too Little, Not Too Much
Balanced feeding
Corals get energy from light and from food. LPS corals like hammers, torches, and acans respond well to small, meaty foods 2 to 3 times per week. Soft corals and filter feeders benefit from fine particle foods and live or preserved phytoplankton. SPS often do best with regular fish feeding, clean water, and occasional broadcast feeds of small particles.
Avoid the nutrient roller coaster
Overfeeding spikes nitrate and phosphate, causing sulking and algae blooms. Underfeeding can cause pale, thin tissue and low nutrient numbers. Keep dosing and feeding steady. If nutrients bottom out, increase feeding slightly or shorten photoperiod while slowly restoring balance.
Pests, Disease, and Predators
Common pests to watch
Flatworms, nudibranchs, and certain crabs or shrimp can irritate corals. Zoanthid-eating nudibranchs hide well. Acropora-eating flatworms and red bugs cause poor polyp extension and tissue loss in SPS. Inspect new corals closely and use a coral dip before adding to your display.
Disease and tissue loss
Brown jelly disease often affects Euphyllia corals and looks like a brown, gooey film. Siphon it out carefully, remove badly infected heads, and dip the remaining healthy tissue with an iodine-based or broad-spectrum coral dip. STN (slow tissue necrosis) and RTN (rapid tissue necrosis) in SPS can follow big parameter swings. Improve stability, add flow, and consider fragging away healthy tips to save pieces.
Fish that nip
Some angelfish, butterflyfish, and even hungry clownfish can annoy corals. If a coral always opens when lights are off but closes when you watch the tank, observe quietly and see if a fish is the cause. Target feeding fish can reduce nipping behavior.
Handling, Shipping, and Acclimation Stress
Good arrival routine
When a coral arrives, float the bag to match temperature for 15 to 20 minutes. Test salinity in the bag so you know the difference. If the coral has been in the bag a long time, do not drip for hours because waste can turn to toxic ammonia once opened. Instead, quickly match temperature, gently acclimate salinity over 15 to 30 minutes, and discard shipping water.
Dip and inspect
Use a proven coral dip according to the label to remove pests. Rinse in clean saltwater before placing in the tank. Mount the coral securely so it does not fall into sand or onto neighbors. Start in lower light and moderate flow, then move up over one to two weeks.
Hands and tools
Always rinse hands to remove soaps or lotions before working in the tank. Use clean tools and avoid cross-contaminating multiple systems. Glue and epoxy are safe when cured, but use sparingly and allow them to set before strong flow hits.
Maintenance Habits That Prevent Sulking
Testing schedule
Test alkalinity two to three times per week in growing reefs, and calcium, magnesium, nitrate, and phosphate weekly until stable. Once you know consumption, you can space tests out but still check regularly. Keep a simple log so you can spot trends.
Water changes and RO/DI
Use RO/DI water with TDS near 0. Change 10 to 15 percent weekly or 15 to 20 percent every two weeks in most new tanks. Match salinity, temperature, and alkalinity of new water to your display to avoid shocks.
Equipment care
Clean pumps and powerheads every month or two to keep flow consistent. Replace activated carbon often. Make sure heaters are not sticking on or off. Inspect magnets and metal parts for rust.
Troubleshooting Scenarios
New hammer coral deflates after placement
Likely causes are too much flow, too much light, or shipping stress. Move it to moderate, indirect flow so the flesh does not rub on skeleton. Place low to mid height with moderate light and increase over a week. Check alkalinity and nitrate; if both are very low, slightly increase feeding and keep alk steady around 8 dKH. Expect improvement within a few days.
Acropora with no polyp extension after light upgrade
Assume light shock. Reduce intensity by 30 to 40 percent, use acclimation mode, and increase over two to three weeks. Verify alkalinity is not swinging and that nitrate and phosphate are not near zero. Increase random flow. Polyp extension should slowly return if tissue is intact.
Zoanthids closed for days
Look for pests first: nudibranchs or sundial snails. Dip the colony and inspect with a flashlight. Ensure no detritus is trapped. Check for shadowing or very low flow. Test for elevated phosphate or a sudden alkalinity shift. After removing the cause, zoas may take a few days to reopen.
When to Frag, Move, or Wait
Fragging to save tissue
If an LPS head is infected with brown jelly, remove that head and dip the remaining healthy ones. For SPS with RTN, frag small healthy branches away from dead tissue and place them in clean flow. Do not frag soft corals for stress alone unless there is clear rot.
Moving vs. holding steady
Move a coral if flow is obviously wrong, if a neighbor is stinging, or if light is clearly too strong. If parameters are off, fix water first, then consider moving. Avoid moving corals many times in a short period. Repeated moves can keep them sulking.
Common Mistakes That Make Corals Sulk
1) Rapid changes to light or flow without acclimation. 2) Chasing “perfect” numbers and causing swings. 3) Letting salinity drift due to poor top-off. 4) Overfeeding and then over-correcting with aggressive nutrient removal. 5) Adding many corals too fast to a young system. 6) Skipping quarantine or dips and introducing pests. 7) Placing incompatible corals too close, leading to stings or chemical warfare. 8) Ignoring equipment maintenance and allowing pumps or heaters to fail. 9) Using tap water or high-TDS RO/DI, importing contaminants. 10) Trying several fixes at once, making it hard to know what worked and potentially stacking stress.
A Simple Action Plan When a Coral Sulks
First 0 to 24 hours
Check temperature and salinity. Test alkalinity, nitrate, phosphate, and ammonia. Reduce light intensity 20 to 30 percent if a recent change occurred. Improve aeration and ensure surface agitation. Look for stings, pests, or direct flow blasts and correct them. If ammonia or nitrite is present, perform a 20 to 30 percent water change and add extra air.
24 to 72 hours
Stabilize parameters and keep hands out of the tank. Run fresh activated carbon and consider a Poly-Filter if contamination is suspected. Adjust flow to be more random and indirect. If the coral shows tissue rot, isolate and dip appropriately, and consider fragging clearly healthy parts away from dead tissue.
1 to 2 weeks
Slowly return lighting toward your target using acclimation. Maintain a steady feeding routine without big swings. Keep logs of test results and observations. If multiple corals remain unhappy, send an ICP test, re-check RO/DI, and review salt mix consistency. Avoid drastic moves unless you discover a clear cause.
Setting Parameter Targets by Coral Type
Softies focus
Temperature 25 to 26°C, salinity 1.025 to 1.026, alkalinity 7.5 to 9 dKH, calcium 400 to 440 ppm, magnesium 1280 to 1350 ppm. Nitrate 5 to 15 ppm and phosphate 0.05 to 0.1 ppm are often tolerated. PAR 50 to 150 with gentle to moderate flow.
LPS mixed reef
Temperature 25 to 26°C, salinity 1.025 to 1.026, alkalinity 8 to 9 dKH, calcium 420 to 450 ppm, magnesium 1300 to 1350 ppm. Nitrate 5 to 15 ppm, phosphate 0.03 to 0.08 ppm. PAR 75 to 200 with moderate, indirect flow.
SPS leaning
Temperature 25 to 26°C, salinity 1.026, alkalinity 7.5 to 8.5 dKH held very stable, calcium 420 to 450 ppm, magnesium 1300 to 1400 ppm. Nitrate 5 to 10 ppm, phosphate 0.02 to 0.05 ppm. PAR 200 to 350 with strong, random flow and high oxygen.
Preventing Sulk With Smart Tank Design
Space and layout
Leave space for growth and for sweeper tentacles. Place defensive corals down-current from sensitive ones. Use rock structures that allow you to adjust height and light easily.
Redundancy and monitoring
Use two smaller heaters instead of one large one. Consider a temperature controller. Battery air pumps or a UPS for critical flow can prevent oxygen drops during outages. If possible, use a simple controller or timers to keep routines steady.
When a Water Change Is the Best Answer
Reset without shock
When the cause is unclear, a well-matched 20 to 30 percent water change can dilute toxins, stabilize pH, and increase oxygen. Make sure new water matches salinity, temperature, and alkalinity to avoid adding more stress. Follow with fresh carbon and a small increase in surface agitation.
How to Know a Coral Is Recovering
Positive signs to look for
Polyp extension increases, flesh re-inflates on LPS, and colors look richer rather than pale or muddy. The coral stops producing excess slime. The base of SPS looks tight and not receding. Growth tips regain shine. These changes may take days to a couple of weeks depending on how severe the stress was.
Introduction to Testing Tools for Clarity
Reliable kits and checks
Use fresh, quality test kits and confirm odd results with a second brand when possible. Calibrate refractometers and clean glass vials. ICP testing can be helpful when problems are deeper than basic tests show, such as metal contamination or ionic imbalance.
Bringing It All Together
Corals sulk for a reason, and water quality is at the heart of most causes. Keep temperature and salinity steady, hold alkalinity in a safe range, avoid nutrient extremes, and make changes slowly. Acclimate to light and flow with patience. Use carbon to guard against chemical warfare and keep your equipment clean and reliable. Dip new corals, watch for pests, and give each coral the right placement and space.
When a coral sulks, think like a detective. Start with simple checks, stabilize the basics, and adjust one thing at a time. Most corals recover if you catch issues early and avoid big swings. With steady habits and a calm approach, your reef will show you less sulk and more growth, color, and movement. Happy reefing, and may your corals look content, not cranky.