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A coral can look healthy for months or even years, then crash seemingly overnight. When that happens, it feels confusing and discouraging. The good news is that sudden coral loss almost always has a cause you can find, fix, and prevent. In this guide, you will learn what “sudden death” really looks like, the most common triggers, fast triage steps to protect the rest of your reef, and simple ways to make your system more stable for the long term. The goal is easy-to-understand advice you can use today, even if you are new to saltwater tanks.
What “sudden death” really looks like
SPS, LPS, and soft corals fail in different ways
Not all corals show trouble the same way. Recognizing the pattern helps you guess the cause.
SPS (small polyp stony) corals, like Acropora and Montipora, often go from fine to bad very fast. Tissue can peel off in hours (called RTN, rapid tissue necrosis) or slowly over days (STN). Color may fade to pale or white (bleaching), but the skeleton stays. SPS are very sensitive to fast changes in temperature, alkalinity, light, and contaminants.
LPS (large polyp stony) corals, like Euphyllia, Trachyphyllia, and Acanthastrea, often show recession at the base or between heads. A brown, slimy film can form (brown jelly disease). Flesh may pull away from the skeleton or heads may bail out. LPS are sensitive to swings in salinity and alkalinity, bacterial infections, low oxygen, and pests.
Soft corals and zoanthids rarely bleach in a classic way. Instead, they stay closed, shrink, melt, or detach. They can be harmed by toxins from other corals, chemical contamination, and pests like nudibranchs. They tolerate nutrient swings better than SPS but still fail when parameters swing hard.
Timing offers clues
When the decline starts can point to the cause. A crash right after a change (like cleaning, dosing, a new light schedule, or new equipment) suggests that change is the trigger. Coral loss at night suggests low oxygen or pH drops. A collapse after a big water change may point to temperature, salinity, or alkalinity mismatch. A decline after adding a new fish may mean nipping or pest introduction.
The usual suspects: big, fast changes
Temperature spikes or drops
Corals like steady temperatures, usually between 24–26°C (75–79°F). A heater failure, stuck chiller, hot day, or closed lid can swing temperature several degrees in a few hours. SPS can start to slough tissue within hours of a big spike or drop. LPS may look very deflated and then develop brown jelly. Even a short period at 30°C (86°F) can cause bleaching, and a drop below 22°C (72°F) can shock sensitive species.
Tip: Use at least two temperature probes or a probe plus a separate thermometer. Alarms and simple fans on a controller can save a tank.
Salinity mistakes and ATO mishaps
Salinity changes quickly when an auto top-off (ATO) fails, when a reservoir runs dry, or when you mix new saltwater at the wrong concentration. A refractometer that is not calibrated with 35 ppt solution (not RO water) can mislead you. A sudden drop from 35 ppt to 31–32 ppt or a jump to 38–40 ppt can kill sensitive corals, especially LPS, within a day or two.
Tip: Calibrate your refractometer monthly with 35 ppt solution. Check salinity before and after large water changes. Never mix freshwater and saltwater in the display.
Alkalinity swings and ionic imbalance
Alkalinity (dKH) supports coral skeleton building and buffers pH. Rapid swings are a top cause of SPS loss. Jumping from 7 dKH to 10 dKH in a day, or dropping a point or two quickly, can start RTN or STN. Dosing mistakes, new salt mixes, clogged dosing lines, or turning off a calcium reactor without adjustment can cause big changes. Low magnesium can worsen instability by allowing calcium carbonate to precipitate out of the water.
Tip: Change alkalinity slowly, no more than 0.5 dKH per day if you can. Test daily when changing dosing or salt brands.
Light shock or sudden shading
Corals adapt to light slowly. If you raise intensity or change spectrum too fast, you can get light shock. Signs include bleaching on top surfaces and polyp retraction. On the other hand, sudden shading from algae growth, a moved rock, or dirty lenses can starve corals of light. Both extremes can lead to tissue loss, especially for SPS.
Tip: When you change lights or move corals up, use acclimation mode and increase intensity over 2–3 weeks. Clean lenses and splash guards monthly.
Flow changes and low oxygen at night
Good flow brings oxygen and food and removes waste. A pump failure or a change in aquascape that blocks flow can create dead zones. LPS often get bacterial infections when flow is too low. Big colonies can also shade and block flow to their base, causing tissue to die back.
At night, photosynthesis stops and oxygen drops. A sudden fish load increase, a bacteria bloom, or shutting off the skimmer overnight can push oxygen too low, causing coral stress by morning. Euphyllia and SPS show this quickly.
Tip: Keep at least one powerhead on a battery backup. Do not run all circulation off a single power strip. Aim for random, moderate-to-strong flow for most SPS and moderate, indirect flow for LPS.
Water quality and nutrients
Ammonia and nitrite from accidents
A dead fish trapped under rock, an overfeeding event, or a filter crash can release ammonia. Even a small ammonia spike can burn coral tissue, leading to rapid decline. Nitrite is less harmful to marine fish and corals than freshwater, but any ammonia above 0.1 ppm is a red flag.
Tip: Keep a reliable ammonia test on hand. If you suspect a spike, use fresh carbon, perform a measured water change, and increase aeration.
Ultra-low nutrients can starve corals
Zero nitrate and zero phosphate sound good but can be harmful. In very “clean” systems, corals can pale, stop growing, and become fragile. A sudden drop in nutrients after adding more media, increasing skimming, or increasing water changes can trigger bleaching and tissue loss.
Tip: For mixed reefs, aim for nitrate around 2–15 ppm and phosphate around 0.03–0.1 ppm. Feed fish and corals consistently. Adjust export slowly.
Phosphate extremes and precipitation
Very high phosphate can weaken skeletons and encourage algae that smothers corals. Dropping phosphate too fast with a large dose of remover can also shock corals. Using kalkwasser or high-alkalinity salt without balancing calcium and magnesium can cause precipitation (snow-like particles), stealing alkalinity and calcium and stressing corals.
Tip: Change phosphate gradually. Test before and after using removers. Keep magnesium 1250–1400 ppm to support stability.
pH swings from CO2 and dosing
pH naturally drops at night. Heavy room CO2 (closed windows) can push pH low for days. Large bolus doses of alkalinity can spike pH, then crash it later. Corals handle a pH range of about 7.8–8.4, but fast changes are stressful.
Tip: Spread alkalinity dosing out over many small doses. Add outside air to your skimmer or use a CO2 scrubber if indoor CO2 is high. Good surface agitation helps.
Contaminants and toxins
Metals from rusting or cracked gear
Magnets that rust, broken heaters, corroding pumps, and cheap clamps can leak iron, copper, or other metals. Even small amounts can harm sensitive inverts. You may see sudden retraction, unexplained tissue loss, and snails dying at the same time.
Tip: Inspect magnets and pumps every few months. Use a poly-filter pad to detect and remove metals. An ICP test can confirm contamination.
Household chemicals and dirty hands
Air fresheners, bug spray, cleaning sprays, paint fumes, and even residue from hand lotion or sunscreen can enter your tank. These can cause corals to slime up, close, and die within hours.
Tip: Keep lids on during home projects. Wash and rinse arms before tank work. Do not spray anything in the same room as the tank. Use activated carbon after any possible exposure.
Plastics, adhesives, and salt mix issues
Uncured epoxy, cheap plastic containers, or a bad batch of salt mix can cause sudden problems. Poorly mixed saltwater (not fully dissolved, wrong temperature) can shock corals during water changes.
Tip: Use reef-safe adhesives and give them time to cure. Mix saltwater at least 24 hours with heat and flow. Use a food-grade reservoir. If a problem starts right after a new bucket of salt, test a fresh batch and consider switching.
Rock, sand, and old tank syndrome
Older tanks can collect organics in the sand and rock. A deep cleaning that stirs a lot of detritus can release nutrients and hydrogen sulfide. This can burn coral tissue quickly. Rock can also bind phosphate and release it later, fueling algae that smothers corals.
Tip: Vacuum sand in small sections over weeks, not all at once. Rinse filters after cleaning. Run carbon after major maintenance.
Biological causes
Pests and predators
Flatworms on Acropora (AEFW), nudibranchs on Montipora or zoanthids, vermetid snails, parasitic isopods, or a new fish that nips can all cause sudden decline. Look for bite marks, missing tissue patches, eggs on the underside, or corals that close only during certain times (like at night when pests are active).
Tip: Use a coral dip to check for pests. Quarantine new corals. Watch new fish closely; some “reef safe” fish still sample fleshy corals under stress.
Bacterial diseases: RTN/STN and brown jelly
RTN/STN often follows stress like parameter swings. Brown jelly is a bacterial infection that attacks LPS, turning tissue into brown slime that spreads fast. If you see stringy, brown muck on a head, act quickly to cut away affected heads and improve flow and oxygen.
Tip: For RTN on SPS, frag away healthy tips beyond the recession line and increase stability. For brown jelly, siphon the jelly, dip, and improve flow. Reduce organics with carbon and a measured water change.
Algae, cyanobacteria, and dinoflagellates
These can smother coral tissue, block light, and release toxins. Dinoflagellates often bloom in ultra-low nutrient systems and can kill snails and irritate corals. Cyano forms mats that suffocate coral edges. Rapid growth over a coral is a clue.
Tip: Balance nutrients, increase biodiversity, and improve flow. Do not black out the tank without addressing the root cause. Manual removal plus stability works best.
Coral warfare and allelopathy
Corals fight for space with sweeper tentacles and chemicals. A torch coral can sting a neighbor inches away. Soft corals like leathers release compounds that can stress SPS. A new coral placed too close can trigger sudden decline in the loser.
Tip: Give corals room to grow. Run activated carbon and change it regularly in mixed reefs. Watch for nighttime sweepers and adjust spacing.
Human actions that backfire
Overzealous cleaning
Scrubbing rocks, blasting detritus, or cleaning a sump too thoroughly can release a lot of waste into the water. Removing too much biofilm or too many sponges at once can reduce your tank’s filtration ability. Corals may look fine for a day and then crash as ammonia or bacteria shift.
Tip: Clean in stages. Rinse filter socks and media outside the tank. Vacuum a little sand each week, not all at once.
Large, mismatched water changes
A big water change can help in emergencies, but if new water is too cold, too hot, lower or higher salinity, or has very different alkalinity, you can harm corals. If your tank runs at 7 dKH and your new salt is 11 dKH, a 50% change is a big shock.
Tip: Match temperature, salinity, and alkalinity closely. For routine maintenance, smaller, more frequent changes are safer.
Dosing errors and carbon dosing
Overdosing alkalinity, calcium, magnesium, amino acids, or trace elements can cause toxicity or imbalance. Carbon dosing (like vodka, vinegar, or nitrate/phosphate reducers) can drop nutrients too quickly or cause bacterial blooms that lower oxygen.
Tip: Make one change at a time and test after each adjustment. Use dosing pumps only after manual dosing has proven stable. Start carbon dosing slowly and monitor oxygen and nutrients.
Moving or fragging stress
Re-gluing a coral, trimming it, or moving it higher into light and flow can shock it. Gluing over living tissue or placing epoxy right on a base can smother it. A stressed coral is also more likely to catch bacterial infections.
Tip: Plan moves. Handle corals gently. Place epoxy on rock, not on the coral’s tissue. Use light acclimation and moderate flow after moving.
Fast triage: what to do in the first 60 minutes
Stay calm and gather facts. First, look for equipment failures: heater, return pump, powerheads, skimmer, ATO. Restore flow and temperature immediately if needed. Add an air stone or point powerheads at the surface to boost oxygen.
Test the basics with trusted kits: temperature, salinity, alkalinity, and ammonia. If something is clearly off, correct it slowly. For example, if salinity is low, raise it over hours, not in one big jump. If ammonia is present, use fresh carbon, dose a detoxifier if you have one, and do a measured water change.
Run fresh activated carbon and change filter socks. Carbon can remove many toxins and soften chemical warfare. If you suspect metal contamination, add a poly-filter pad.
Physically protect neighbors. Siphon away brown jelly and move healthy corals away from stinging sweepers. If a coral is experiencing RTN, consider fragging healthy tips to a separate rack in moderate flow.
Deeper diagnostics over the next 24–72 hours
Verify your tools. Calibrate your refractometer with 35 ppt solution, double-check temperature with a second thermometer, and compare alkalinity with a different test kit or at a store. Bad readings lead to bad decisions.
Check for contaminants. Inspect magnets, pumps, and heaters for rust or cracks. Smell the tank and sump for unusual odors like solvents or rotten eggs. If you suspect metals or unusual toxins, consider sending an ICP test and doing moderate water changes while running carbon and poly-filter.
Watch at night. Observe the tank 1–2 hours after lights out. Is oxygen low (fish gasping at the surface)? Are sweepers stinging neighbors? Are pests more visible? Does pH drop more than 0.3 overnight? Simple observations can reveal the cause.
Review recent changes. Make a list: new livestock, new food, new salt, dosing changes, cleaning, home projects, or weather heat waves. The cause is often something in the last week.
Targeted fixes based on the diagnosis
If temperature was the issue
Stabilize with a reliable heater controller and a backup heater. Add a small fan or chiller if the room heats up. Set alerts on your controller or use a simple temperature alarm.
If salinity was off
Fix the ATO, top off with the correct water, and match salinity during water changes. Calibrate instruments monthly. Avoid chasing numbers; aim for consistent 34–35 ppt.
If alkalinity swung
Adjust dosing to maintain a stable daily schedule. Change alkalinity by no more than 0.5 dKH per day. Match your salt mix to your target dKH, or do smaller, more frequent water changes.
If light was too strong or too weak
Use light acclimation. Reduce intensity by 10–20% and increase slowly over 2–3 weeks. Consider moving the coral lower temporarily. Clean lenses and ensure even coverage.
If flow or oxygen was low
Reposition powerheads for random, turbulent flow. Avoid blasting a coral directly. Increase surface agitation and keep the skimmer running. Consider a small battery backup for at least one pump.
If nutrients were zero or very high
For zero nutrients, feed more and reduce export slowly. Stop or cut back carbon dosing. For very high nutrients, increase export gradually with more water changes, refugium, or media, but avoid big sudden drops.
If contamination was likely
Run fresh carbon and poly-filter. Do a few 15–25% water changes over several days while matching parameters closely. Remove rusting items. Improve room air quality and avoid sprays.
If pests or disease were present
Dip affected corals and manually remove pests. Frag healthy tissue away from RTN lines and discard infected parts. For brown jelly on LPS, siphon the jelly, cut off infected heads, and increase flow. Quarantine new corals in the future to prevent reintroduction.
Prevention checklist for long-term stability
Build stability into your routine
Test the basics weekly: temperature, salinity, alkalinity, calcium, magnesium, nitrate, and phosphate. Keep a simple log. Make changes slowly and one at a time so you can see the effect. When in doubt, choose stability over “perfect numbers.”
Quarantine and dip new corals
A small quarantine tank with a light and flow can save your display. Dip corals to remove pests and observe for a couple of weeks. Inspect at night with a flashlight for eggs and hitchhikers.
Maintain and inspect equipment
Clean pumps and skimmers regularly. Replace worn O-rings and check magnets for swelling or rust. Keep spares for critical items like heaters and powerheads. Use drip loops and separate power strips to avoid a single point of failure.
Calibrate and verify instruments
Calibrate refractometers with 35 ppt solution, not RO water. Cross-check temperature with a second thermometer. Compare alkalinity readings with another kit or a trusted store every so often.
Plan for power and heat events
A small battery backup for a powerhead can protect oxygen levels for hours. Have frozen water bottles for cooling and an extra heater for cold nights. Set alarms on a controller or smart plug where possible.
Feed and export in balance
Set a simple feeding plan and nutrient export plan and stick to it. Avoid big swings in skimming, media use, and water changes. If you need to change something, do it gradually over weeks.
Place corals with room to grow
Give space between aggressive corals. Research typical sweepers and growth rates. Use carbon regularly in mixed reefs to reduce chemical warfare.
FAQ: quick answers to common questions
Why did one coral die while others look fine?
Different species have different sensitivities. An SPS may die from a small alkalinity swing that soft corals ignore. Also, placement matters. One coral might be in a dead zone for flow or light or be in reach of a stinging neighbor.
Should I do a huge water change right away?
Only if there is clear contamination or ammonia. Large, mismatched changes can make things worse. In most cases, do a moderate change with well-matched water and run carbon while you confirm the cause.
How do I stop RTN on an Acropora?
Improve stability first. Frag healthy tips well past the dying edge and place them in clean, moderate flow with stable light. Sometimes the colony cannot be saved, but frags can survive and regrow.
Is zero nitrate and zero phosphate good?
No. Corals need some nutrients. Aim for low but measurable nitrate and phosphate. Zero nutrients often lead to pale corals, dinos, and instability.
Could my house air be hurting my reef?
Yes. High indoor CO2 lowers pH. Sprays, paint fumes, and cleaners can contaminate water. Improve ventilation, avoid sprays near the tank, and consider a skimmer outside-air line or a CO2 scrubber.
Conclusion
When a well-established coral dies suddenly, it feels like there was no warning. In reality, most crashes trace back to a fast change, contamination, or a hidden biological problem that you can uncover with a simple checklist. Start by restoring the basics: stable temperature, salinity, alkalinity, oxygen, and clean water with carbon. Then look for clues in recent changes, equipment, and nighttime behavior. Make adjustments slowly and deliberately. With a calm approach and good habits, your reef will become steadier, your corals will recover, and future surprises will be far less common.
Reefkeeping is not about chasing perfect numbers; it is about keeping things steady and making small, thoughtful improvements. Focus on stability, quarantine new additions, verify your tools, and plan for common risks. Do that, and the rare sudden loss becomes a lesson that makes your reef stronger, not a setback that keeps you from enjoying the hobby.