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Corals rarely die “for no reason.” When a coral colony suddenly turns white, peels, melts, or stops opening, it is usually the final stage of a problem that has been brewing for days or weeks. The good news is that most causes are common and fixable once you know what to look for. In this beginner-friendly guide, I will walk you through the most frequent reasons corals crash, how to diagnose what went wrong, and the exact steps you can take to save what you can and prevent it from happening again.
Understanding “Sudden” Coral Death
What “sudden death” really looks like
Corals can fail in different ways. Rapid Tissue Necrosis (RTN) looks like tissue peeling off quickly, sometimes within hours, often starting from the base or a damaged edge. Slow Tissue Necrosis (STN) is the same process but over days to weeks. Bleaching is when a coral turns white but the tissue remains, caused by extreme light or temperature stress; some bleached corals can recover if conditions improve. Brown jelly disease appears as a brown, slimy mass that consumes LPS corals, especially euphyllia, very quickly. Each pattern hints at the problem and helps you choose the right fix.
Why it rarely happens out of nowhere
Most “sudden” deaths are the result of instability. Parameters swing, pests quietly multiply, a pump clogs, or a dosing error slowly softens a coral’s resilience until a small trigger sets off a rapid crash. That is why your goal is not perfection but stability. The fastest way to stop losses is to stabilize what the coral experiences: temperature, salinity, alkalinity, nutrients, light, and flow.
Water Parameters and Stability
Temperature swings
Corals hate sudden temperature changes. Even a 2–3°C (3–5°F) swing in a day can trigger RTN or bleaching. Aim for a stable 24–26°C (75–79°F). Use a reliable heater with a controller, avoid placing the tank in a sunlit or drafty area, and consider a small fan or chiller if your room overheats. If your tank overheats one day and cools the next, you will eventually see coral stress even if numbers look “acceptable.”
Salinity changes
Salinity shock is a top reason for rapid coral decline. Auto top-off failures, mixing salt too fast, or doing a large water change with mismatched salinity can cause sudden stress. Keep salinity at 35 ppt or 1.025–1.026 specific gravity and match new water within 0.001 SG. Use a calibrated refractometer and avoid big jumps. If you discover a salinity problem, correct it slowly over hours, not instantly.
Alkalinity, calcium, and magnesium stability
Alkalinity stability is critical. Choose a target range such as 7.5–9.0 dKH and hold it there. Rapid alk changes, especially downward, are a classic trigger for STN in SPS corals. Calcium should be around 400–450 ppm and magnesium 1280–1400 ppm. Dosing mistakes, inconsistent two-part dosing, and clogged lines can swing these values. If you adjust, do so gradually. When in doubt, stop aggressive dosing and perform measured water changes to reset safely.
Nutrient levels: nitrate and phosphate
Ultra-low nutrients can starve corals; high nutrients can fuel harmful bacteria and algae. For a mixed reef, many successful tanks hold nitrate around 5–15 ppm and phosphate around 0.03–0.10 ppm. The danger is not just the number, but how fast it changes. Dropping phosphate too quickly with aggressive GFO can trigger RTN. Suddenly stripping nitrate with carbon dosing can spark bacterial imbalances. Adjust media and dosing slowly, test often, and aim for small movements over days, not hours.
pH and CO2 swings
pH swings with the light cycle, often dipping at night. Corals tolerate a range of about 7.9–8.3, but larger swings can stress delicate species. Poor room ventilation raises CO2, lowering pH and oxygen. Improving surface agitation, increasing flow, sending skimmer air from outside, or using a CO2 scrubber on the skimmer intake can stabilize pH. A flat pH trace is often a sign of stable oxygen and CO2 levels, and stable corals.
Light-Related Causes
Light shock and acclimation
Corals can bleach in hours if exposed to more PAR than they are used to. Even moving a coral up a few inches or switching light brands can double PAR. Always acclimate corals to light gradually. Use your light’s acclimation mode or reduce intensity for one to two weeks, increasing a little every few days. Start new frags low and shaded, then move up if they respond well.
Dirty lenses, old bulbs, and sudden changes
Salt creep and dust can reduce light output, so cleaning LED diffusers and lenses periodically is important. Changing old T5 bulbs or swapping LED fixtures can cause abrupt increases or spectrum shifts that corals are not ready for. Treat any lighting change as a new light and acclimate again to avoid shock.
Photoperiod and spectrum
Most reefs do well with a total photoperiod of about 8–10 hours of strong light, plus a gentle ramp up and down. Blue-heavy spectrums are generally safer for acclimation than full-intensity white. If you need to push growth later, do it slowly. If corals pale while still extended, light may be too strong. If they brown and stretch, light may be too weak or nutrients too high.
Flow-Related Causes
Too little flow
Corals need consistent, random flow to bring food and oxygen and remove waste. Dead spots collect detritus and can lead to tissue recession at the base of SPS and around LPS skeleton edges. If a pump slows or clogs, you may see issues appear first on the side with poor flow. Adjust pumps to create crosscurrents and gently sway coral flesh without blasting it.
Too much direct flow
Blasting a coral with a narrow jet can strip tissue or cause constant retraction. Euphyllia, brains, and fleshy LPS can get torn by harsh flow. If you see a coral unwilling to open where a pump points, diffuse the flow, pulse it, or redirect it to bounce off glass. Aim for turbulent, alternating patterns rather than constant one-direction flow.
Pump maintenance
Pumps lose output as they accumulate biofilm and calcium crust. Clean wave makers and return pumps regularly to maintain steady flow and oxygen. A pump failure can be enough to tip a stressed tank into a coral crash, especially overnight when oxygen is naturally lower.
Contaminants and Toxins
Metals and household chemicals
Rusting magnets, corroded clamps, and cheap tools can leach metals such as iron, copper, and zinc. Aerosol sprays, paint fumes, or cleaners used near the tank can enter the water and irritate corals. Always wash hands without soap or lotion before working in the tank. Use aquarium-safe tools, avoid spraying air fresheners or bug sprays in the room, and replace rusted parts promptly. If you suspect contamination, run fresh activated carbon and consider a Poly-Filter pad, which can absorb a broad range of toxins and change color to indicate metal presence.
Chemical warfare between corals
Soft corals and leathers can release allelopathic chemicals that suppress SPS and LPS, especially in smaller tanks. Intense warfare can cause polyp retraction and slow decline that suddenly tips into RTN. Running high-quality carbon, performing regular water changes, and giving space between species helps. If a leather recently spawned or was fragged, increase carbon and water changes temporarily.
Bad water and salt issues
Using tap water, or RODI with a failing membrane or exhausted resin, can introduce ammonia, chloramines, or heavy metals. Keep TDS near zero and change filters as needed. In rare cases, a bad batch of salt can cause problems; mixing and aerating new saltwater for 24 hours, and testing it before use, reduces risk. Always match temperature, salinity, and pH when doing larger changes.
Pests, Predators, and Diseases
Coral-specific pests
Acropora-eating flatworms, Montipora-eating nudibranchs, zoanthid-eating nudibranchs, and red bugs can quietly multiply until damage becomes obvious. Look for bite marks, missing flesh between polyps, small white egg clusters, or dull, irritated polyps that never extend. Dips can reveal pests, but eggs often survive and require multiple treatments or quarantine. If only one coral type declines while others thrive, think species-specific pests.
Brown jelly disease and bacterial issues
Brown jelly is a fast-moving bacterial infection that attacks LPS, especially euphyllia, torches, and hammers. You will see brown, gooey mass on the tissue that can spread to neighboring heads. Quick action is needed: siphon the jelly away, frag off healthy heads well into clean tissue, perform an iodine dip on the affected pieces, and increase flow to prevent re-accumulation. Stabilizing parameters and improving oxygen helps corals fight infections.
Fish and invertebrate predators
Many “reef-safe with caution” fish nip corals enough to cause stress and slow death. Angels, butterflies, some wrasses, and even tangs can sample fleshy LPS or pick at polyps. Crabs, certain starfish, and pests like aiptasia anemones can cause localized damage that looks like disease. If a coral recedes on one side only or closes during the day but opens at night, watch the tank quietly from a distance to catch the offender.
Tank Age and Maturity
New tank syndrome
Brand-new systems lack biodiversity and stability. While ammonia and nitrite may read zero, the microbiome that supports coral health is still forming. SPS especially can struggle in immature tanks with low and unstable nutrients. It is often smarter to start with hardy soft corals and LPS, let the tank mature for several months, and add sensitive species later.
Bacterial imbalances and blooms
Sudden changes in nutrients or overuse of carbon dosing can cause bacterial blooms that lower oxygen and irritate corals. Dinoflagellates thrive in ultra-clean systems, smothering coral tissue and stinging polyps. The fix is often counterintuitive: raise nutrients slightly, increase biodiversity, and stabilize pH and oxygen rather than dosing harsh chemicals.
Husbandry and Stress
Handling, fragging, and placement
Rough handling or gluing can crush tissue or introduce infection. Always handle corals by the plug or skeleton, not soft flesh. Avoid shading a coral suddenly by placing it under an overhang if it was previously in bright light. When moving corals, make changes gradually and give them time to adapt to new flow and light patterns.
Feeding and energy balance
Not all corals get enough energy from light alone. Many LPS benefit from targeted feedings once or twice a week with small particle foods or meaty items. Non-photosynthetic corals require frequent feeding and pristine, stable water. If LPS corals shrink and lose flesh while parameters look okay, they may simply be starving.
Media and dosing mistakes
GFO can strip phosphate too fast. Carbon dosing can overshoot and crash nitrate. Skimmers set too wet can remove nutrients rapidly. Any abrupt change in filtration can stress corals. Move slowly: change only one thing at a time, measure results, and let the tank settle before making another adjustment.
Emergency Response: What To Do Today
Measure, log, and stop the swings
Start with accurate tests for temperature, salinity, alkalinity, nitrate, and phosphate. Calibrate your refractometer. Cross-check with a second test kit if results seem off. If any value is far from your target, correct gradually over 24–48 hours. Pause automation that might be overdosing, such as dosers stuck on or carbon dosing if nutrients are already low.
Do a safe, significant water change
A 20–30 percent water change with well-mixed, heated, aerated saltwater matched to your tank’s salinity and temperature can dilute toxins and reset chemistry. Add a fresh bag of high-quality activated carbon in a high-flow area and replace it after a few days. If you suspect chemical contamination, a Poly-Filter pad can help remove metals and unknown pollutants.
Check equipment and oxygen
Confirm the heater actually holds set temperature and does not overshoot. Verify the auto top-off is working and not sticking. Ensure return pumps and wave makers are running at full output. Increase surface agitation to boost oxygen, especially at night. If you have a skimmer, make sure it is functioning and not shut down due to a recent additive or food oil.
Isolate and frag if needed
If one coral is melting from brown jelly or RTN and threatening neighbors, move it to a quarantine container or frag it to save healthy sections. Use a clean tool to cut well into healthy tissue and remount frags on clean plugs. Consider an iodine-based dip for infected pieces. Increase flow around the affected area in the display to keep detritus and bacteria from settling.
Hold steady and observe
Once you have stabilized parameters and added carbon, resist the urge to make multiple changes at once. Observe polyps, tissue edges, and color over the next 24–72 hours. Corals often look worse before they look better after a shock. Stability and patience win far more often than aggressive tinkering.
Root-Cause Troubleshooting
Validate your numbers
Old reagents and human error cause false readings. If results do not match what you see, test again with a new kit or at a local shop. Verify salinity with a calibrated device. Many coral crashes blamed on “mystery causes” turn out to be simple measurement errors.
Watch day and night
Observe the tank before lights on and after lights out. Are fish gasping at the surface? Do corals look better with lights off and worse during the day, which might suggest light shock, or the opposite, which might suggest nighttime oxygen issues? A cheap dissolved oxygen test or continuous pH log can reveal hidden problems.
Consider an ICP test
If problems persist and common causes are ruled out, an ICP test can detect elevated metals or depleted trace elements. While trace dosing can help advanced tanks, beginners should focus first on removing contaminants and restoring stability rather than chasing every element.
Prevention Best Practices
Quarantine and dip new corals
Every new coral should be dipped following the product’s instructions to remove pests, then observed for at least a week if you can. Inspect for eggs and remove them manually. Quarantine prevents a single bad frag from spreading problems to the entire display.
Pick reasonable targets and keep them steady
Good starting targets for a mixed reef are 24–26°C temperature, 1.025–1.026 salinity, 7.5–9.0 dKH alkalinity, 400–450 ppm calcium, 1280–1400 ppm magnesium, 5–15 ppm nitrate, and 0.03–0.10 ppm phosphate. The most important word is steady. If you must move a value, move it slowly.
Map light and flow
If possible, borrow a PAR meter to understand how much light different spots receive. Place light-demanding corals where PAR fits their needs and start lower than you think. Set up flow to create varied, indirect movement rather than one strong jet. Clean pumps and light lenses regularly to keep output consistent.
Plan maintenance and backups
Make a simple schedule for filter changes, skimmer cleaning, and pump maintenance. Keep spare heater and a small air pump with battery backup for power outages. Label doser containers to prevent mix-ups. Many disasters are avoided with a little preparation.
Limit big, fast changes
Change one thing at a time, measure, and wait. Whether adjusting GFO, switching salt, or changing light settings, gentle and gradual beats fast and drastic. Give corals time to adapt and you will see more growth and less stress.
Quick Fixes Versus Long-Term Stability
When to use carbon, GFO, UV, and bacteria
Fresh activated carbon and a water change are safe first-line tools whenever corals look irritated. GFO is useful for high phosphate but should be introduced slowly and kept modest to avoid stripping too fast. UV can help with water clarity and reduce free-swimming pathogens but will not fix poor husbandry. Bottled bacteria can help stabilize new tanks or after big disruptions, but they are not a cure for bad parameters. These tools support, but do not replace, consistent care.
Patience and consistency win
Corals respond to trends, not snapshots. A single perfect test does not undo a week of swings. Focus on making today stable, and then repeat that tomorrow. As your tank matures and you build steady routines, “sudden” coral deaths become rare.
Conclusion
When corals suddenly decline, it is usually a signal that something became unstable or a hidden problem reached a tipping point. Start with the basics: temperature, salinity, alkalinity, nutrients, light, and flow. Look for contaminants, pests, and predation. Act quickly but gently with water changes, carbon, improved oxygen, and, if needed, fragging and dips for infected pieces. Then shift your attention to prevention through quarantine, careful acclimation, and a steady maintenance routine. The path to thriving corals is not about chasing perfect numbers; it is about building a calm, predictable environment where small, thoughtful changes compound into long-term success. With patience and the steps in this guide, you can stop the slide, save what you can, and help your reef rebound stronger than before.