We are reader supported. When you purchase through links on our site, we may earn an affiliate commission. Also, as an Amazon affiliate, we earn from qualifying purchases.
When a coral that has grown well for months suddenly declines, it feels confusing and urgent. Corals rarely die without a cause. Something changed, and your job is to find it fast, stop the damage, and restore stability. This guide shows the common triggers of sudden coral loss, the fixes that work, and the habits that keep reefs steady. If you act quickly and methodically, most setbacks can be contained.
What sudden coral decline looks like
You may see polyps shut for days, color washing out, tissue peeling from the base, or a jelly-like film overtaking fleshy corals. SPS can lose tissue in hours, called rapid tissue necrosis. LPS can melt from bacterial infections like brown jelly disease. Soft corals can collapse from toxins or salinity shocks. These signs point to stress, instability, or infection. The faster you address the root cause, the better the outcome.
Your first 10 minutes: triage that saves colonies
Stabilize the basics
Check temperature and salinity first. Fix anything out of range slowly. Confirm all pumps and the skimmer are running to maintain oxygen. If the room is hot, aim a fan at the water surface and remove the canopy to cool by evaporation. If apparent contamination is suspected, start fresh activated carbon and increase aeration.
Quick interventions that buy time
Perform a 10 to 20 percent water change with well-mixed, temperature and salinity matched saltwater. Siphon detritus around the affected coral to lower bacterial load. If a coral shows brown jelly or obvious infection, gently turkey baste the slime away and consider an iodine-based dip, then place the coral in higher, random flow. If tissue is peeling on SPS, frag a healthy tip above the retreating line and move the frag to a stable area. These steps help while you diagnose the cause.
Root causes and how to fix them
Temperature swings
A swing outside 24 to 26 degrees Celsius stresses coral metabolism and their symbiotic algae. Heaters that stick on, fans that fail, or seasonal heat waves can trigger bleaching or tissue loss. Use a reliable heater controller and a thermometer you trust. If the tank is hot, cool it with fans and reduce photoperiod until stable. Avoid big drops. Aim for slow changes no more than 0.5 degrees Celsius per day.
Salinity instability
Auto top off failure, overdosing salt mix, or a skimmer overflowing can shift salinity. Large jumps can shock softies and LPS first. Confirm with a calibrated refractometer and keep at 35 ppt or 1.025 to 1.026 specific gravity. Fix slowly using a drip method. When salinity is high, remove small volumes of tank water and add equal volumes of pure RO or DI water over hours. When low, add concentrated saltwater gradually. Calibrate testing tools monthly.
Alkalinity and pH shocks
Alkalinity fuels coral skeleton growth and buffers pH. Fast swings from dosing errors are a top coral killer. Aim for 7.5 to 9 dKH and keep daily swing under 0.3 dKH. If alk is off by more than 1 dKH, correct in steps over 24 to 48 hours. pH between 7.8 and 8.4 is fine if stable. In a sealed room, CO2 builds up and pH drops, lowering oxygen at night. Open a window, increase surface turbulence, and run your skimmer air line to fresh air if possible. Focus on stability more than chasing a high pH number.
Nutrient starvation or spikes
Zero nitrate or phosphate can starve corals and their algae, especially SPS. Nutrients that spike from heavy feeding or media failure can fuel pests and bacterial blooms. For most mixed reefs, target nitrate 2 to 15 ppm and phosphate 0.03 to 0.1 ppm. If nutrients are zero, feed a bit more, reduce aggressive filtration, or dose small amounts of nitrate or phosphate products carefully. If nutrients are high, increase export through water changes, stronger skimming, and clean media. Aim for slow corrections to avoid additional stress.
Light shock and spectrum shifts
Upgrading lights, changing settings, or cleaning salt-crusted lenses can double PAR overnight. Bleaching follows. Always acclimate when changing intensity or spectrum. Start at 50 to 70 percent of the previous PAR and increase 5 to 10 percent per week. Keep a consistent photoperiod. If a coral pales under new light, lower intensity, shorten the photoperiod, or move the coral to lower PAR while it recovers.
Flow problems
Corals need strong, random flow to bring oxygen and remove waste. Dead spots trap detritus and promote infections, while direct blasts can tear tissue. After a powerhead moves or dies, animals can suffer within hours. Clean and service pumps monthly so output stays steady. Aim for enough flow that food and mucus do not settle on coral tissue. Adjust pumps to alternate patterns and avoid narrow jets aimed at a single colony.
Contaminants and toxins
Rusting magnets, corroded pump shafts, sprays near the tank, soap residues, and household aerosols can poison corals. Metals and toxins irritate tissue and can turn healthy colonies pale or necrotic. Run fresh activated carbon in a high flow area and consider a poly filter pad that binds a broad range of contaminants. If you use RO or DI water, check the total dissolved solids and replace filters when readings rise. If the cause remains unclear after basic fixes, an ICP test can reveal hidden contaminants and guide targeted removal.
Pests and pathogens
Flatworms, nudibranchs, and certain crabs feast on specific corals. Brown jelly disease is a fast LPS killer after damage or stress. A magnifying glass and a flashlight can show bite marks, eggs, or moving pests. Dip new corals before adding them to the display. If pests are present, use coral dips per label and manually remove eggs. For brown jelly, isolate and dip, then increase flow around the coral and siphon waste. Remove dead tissue quickly so it does not seed the rest of the tank.
Chemical warfare and space conflicts
Soft corals and some LPS release allelopathic compounds that irritate neighbors. At night, sweeper tentacles from LPS can sting nearby colonies. When corals grow closer, chemical stress increases. Run activated carbon and change it regularly. Space aggressive species farther apart and trim overgrowth before it touches. Use frag racks or barriers to keep peace in crowded areas.
Mechanical damage and handling stress
Accidental stings, falls, and rough handling open the door to infection. After aquascape changes or pest inspections, place corals securely in proper flow and ensure no rock can shift onto them. Handle tissue by the base or plug, not by polyps. If damage occurs, dip to reduce bacterial load and keep flow high but not blasting.
Testing routine that prevents surprises
Core parameters and safe ranges
Keep a simple, weekly test habit. Measure temperature daily. Check salinity two to three times per week. Test alkalinity two to four times per week if you have many stony corals, weekly if lightly stocked. Check calcium and magnesium weekly. Measure nitrate and phosphate weekly until stable. Target ranges that match your coral mix and keep them steady.
For mixed reefs, these ranges are workable. Temperature 24 to 26 degrees Celsius. Salinity 35 ppt or 1.025 to 1.026. Alkalinity 7.5 to 9 dKH. Calcium 400 to 450 ppm. Magnesium 1280 to 1400 ppm. Nitrate 2 to 15 ppm. Phosphate 0.03 to 0.1 ppm. Stability matters more than landing a single exact number.
Tools and calibration
Trustworthy data prevents bad decisions. Calibrate refractometers with calibration solution, not RO or DI water, once a month. Cross check test kits against a reference solution or a different brand every few months. Replace expired reagents. If a reading looks odd, test again and verify with another kit or your local store before making big corrections.
Dosing and water change strategy
When to dose and how to avoid overdoses
Dose only what your tests show the tank is consuming. Alkalinity is consumed fastest. Calcium and magnesium change slower. Small, frequent doses are safer than big swings. If you automate dosing, verify the pump output and recalibrate every few months. If a dosing error happens, stop dosing, perform measured water changes, and correct back to target slowly. Avoid stacking multiple corrections on the same day.
Safe water changes that help rather than harm
Mix new saltwater for at least a few hours with heat and flow. Match temperature and salinity before adding. Aim for consistent, modest changes like 10 to 15 percent weekly or biweekly in young tanks, and adjust based on nutrient trends. During emergency changes for contamination or spikes, split into multiple smaller changes over two days to reduce shock.
Lighting and flow tuning by coral type
Soft corals
Soft corals prefer moderate light and moderate, varied flow. PAR around 50 to 150 works well. Keep nutrients detectable and avoid ultra low nutrient conditions. They release chemicals, so run carbon and give them space.
LPS corals
LPS thrive with moderate light, PAR around 75 to 200, and gentle to moderate flow that keeps flesh moving without folding or tearing. Feed small meaty foods once or twice a week. Watch for sweeper tentacles at night and give room to neighbors.
SPS corals
SPS demand stable alkalinity, strong random flow, and higher light, PAR around 200 to 350 depending on species. Avoid rapid changes. Acclimate frags to light over several weeks. Keep nutrients low but not zero. Even hardy species suffer when alk or temperature swings.
Feeding and microbiome balance
Corals get energy from light and from food. LPS benefit from target feeding. SPS capture fine particles. Do not overfeed suddenly, as this can spike nutrients and fuel harmful bacteria. Increase feeding gradually and monitor nitrate and phosphate. A healthy microbiome thrives on stable nutrients, clean flow paths, and consistent export through skimming and water changes. If you smell a swampy odor or see cloudy water, increase aeration and perform moderate water changes to restore oxygen and balance.
Quarantine and dip protocol simplified
Quarantine new corals in a separate tank when possible. Dip with an iodine or commercial coral dip to remove pests, then observe for a couple of weeks. Inspect at night for hidden predators. Move only clean and healthy corals into the display. This prevents introducing pests or pathogens that can spread fast and wipe out established colonies.
Maintenance habits that keep corals stable
Log your tests and changes. Patterns reveal trends before crises. Clean pumps and wavemakers monthly so flow stays steady. Replace carbon every two to four weeks depending on tank size and bio load. Keep RO or DI filters fresh, and watch the total dissolved solids reading. Calibrate tools on a schedule. Verify timers for lights and dosing pumps after power outages. Avoid making many changes at once. When something works, keep it steady.
When to frag or remove a dying coral
If tissue is receding on SPS, frag live parts above the dead line and move to a stable rack with good flow and moderate light. For LPS with brown jelly or rot, siphon, dip, and if needed cut away infected heads to save the rest. Remove fully dead pieces promptly to lower nutrient spikes and bacterial risk. Preserve stability for the healthy majority while you correct the root cause.
Case patterns you can recognize fast
If multiple corals decline at once, think system wide stress. Temperature spike, salinity swing, dosing mistake, or contamination. If one coral or a single species group crashes while others look fine, think pests, allelopathy, flow shadows, or localized light shock. If tissue loss starts after cleaning or equipment changes, check for light intensity changes, stirred detritus, or new metal exposure. Matching the pattern to the likely cause speeds up the fix.
A simple recovery plan
Day one, stabilize temperature and salinity, verify flow and oxygen, and perform a measured water change. Add fresh carbon. Remove decaying tissue and dip infected corals. Reduce light by 10 to 20 percent if bleaching or heat was involved. Day two, test alkalinity, calcium, magnesium, nitrate, and phosphate. Correct in small steps. Improve random flow if detritus collects. Over the week, maintain stable conditions, continue observation, and avoid extra changes. After stability returns, slowly restore light and dosing schedules.
Proof you are back on track
Polyp extension improves, tissue stops receding, color deepens, and new tips or mouths appear. Tests show narrow daily swings. Algae on glass grows at a consistent rate rather than exploding or disappearing. Corals respond to feeding without sliming excessively. Your logbook shows a week or more of steady parameters and normal behavior.
Conclusion
Established corals die suddenly when stability breaks, contaminants enter, or pests and pathogens take advantage of stress. The fix is clear and repeatable. Stabilize temperature and salinity. Restore oxygen and flow. Use carbon and water changes to buffer shocks. Dip or frag as needed. Correct alkalinity and nutrients slowly into safe ranges. Acclimate light and maintain clean, consistent equipment. With a calm approach and steady habits, most tanks recover, and corals return to growing, coloring up, and filling space with healthy tissue.
FAQ
Q: What are the first steps when a coral starts dying suddenly
A: Check temperature and salinity, confirm all pumps and the skimmer are running, add fresh carbon, perform a 10 to 20 percent matched water change, and stabilize parameters before making further changes.
Q: How do I prevent light shock after changing lights or settings
A: Acclimate by starting at 50 to 70 percent of prior intensity and increase 5 to 10 percent per week, while keeping a consistent photoperiod and watching coral color and polyp extension.
Q: What nutrient levels are safe for a mixed reef
A: Aim for nitrate 2 to 15 ppm and phosphate 0.03 to 0.1 ppm, and avoid zero nutrients or sudden spikes by adjusting feeding and export gradually.
Q: How can I tell if a contaminant is the problem
A: Look for rusting magnets or corroded metal, consider household sprays near the tank, run fresh activated carbon, check RO or DI water quality, and use an ICP test if the cause remains unclear.
Q: When should I frag or dip a coral
A: Frag SPS when tissue is receding to save healthy tips, and dip LPS or other corals showing slime or infection like brown jelly, then place them in strong, random flow to recover.