Corals die in reef aquariums for a surprisingly small number of reasons, and most come down to unstable water chemistry, excessive nutrients, inadequate flow, pests, or contamination. The tricky part is that corals rarely give you one obvious signal. Instead, they fade, bleach, or peel apart in ways that look similar across very different root causes. Here’s how to narrow down what’s actually killing your corals.
Unstable Alkalinity Is the Most Common Killer
Alkalinity swings damage corals faster than almost any other parameter problem, and the counterintuitive part is that even correcting your numbers too quickly can cause harm. If you’re seeing white or burnt tips on SPS corals, tissue pulling back from growth points, sudden color loss or browning, or reduced polyp extension, you’re likely looking at alkalinity burn. This happens when alkalinity changes rapidly, whether up or down, and the damage tends to show up within hours to days of a dosing change or water change that shifted the number too fast.
The lesson most reefers learn the hard way: a slightly imperfect but stable alkalinity is far safer than chasing a perfect number with aggressive corrections. If your alkalinity is off, adjust it by no more than 0.5 dKH per day. Corals can adapt to a range of alkalinity levels, but they cannot adapt to volatility.
High Phosphate Starves the Skeleton
Phosphate doesn’t just fuel nuisance algae. It directly interferes with how corals build their calcium carbonate skeletons. Phosphate molecules travel through the gaps between coral cells, reach the internal space where the skeleton forms, and physically adsorb onto the growing crystal surfaces. This disrupts the crystallization process and slows or stops skeletal growth. Research published in Marine Pollution Bulletin found that phosphate concentrations as low as 0.5 micromoles per liter significantly inhibited skeletal formation in coral juveniles, and the damage worsened as more phosphate-laden water flowed over the corals.
That finding is important because it means the total phosphate load matters, not just the concentration at any single moment. A tank with moderately elevated phosphate and high flow can actually deliver more phosphate damage than a tank with higher phosphate and lower flow. In healthy reef environments, phosphate naturally sits below 0.1 micromoles per liter. For your aquarium, that translates to keeping phosphate below roughly 0.03 ppm. Above 0.15 ppm, you’re entering territory where calcification slows noticeably, and concentrations above 0.5 ppm can halt skeletal growth entirely.
Temperature Stress and Bleaching
Corals bleach when they expel the symbiotic algae living in their tissue, the algae that give them both their color and most of their energy. This expulsion is triggered by heat stress. The threshold is specific: just 1°C (about 1.8°F) above the coral’s normal maximum monthly temperature is enough to trigger bleaching. For most reef aquariums kept around 78°F, that means sustained temperatures above 80°F start causing problems, and anything above 82°F for more than a day or two can be lethal.
Bleaching doesn’t kill corals instantly. A bleached coral is stressed and starving, but it can recover if the temperature drops back to normal within a week or two. The danger is prolonged exposure. If your heater malfunctions or your room temperature spikes during summer, check your corals for a pale, washed-out appearance. That’s the earliest visible sign of thermal stress. Cold shock from a chiller malfunction or a large water change with cold water can cause similar damage.
Low Flow Lets Detritus Become Toxic
When water flow is too low, organic debris settles on coral tissue and triggers a surprisingly fast chain reaction. Bacteria in the settled material begin consuming oxygen, creating a low-oxygen, low-pH microenvironment directly on the coral surface. Research from the Proceedings of the National Academy of Sciences showed that organic-rich sediment caused tissue degradation within a single day of settling on corals. The mechanism works in stages: first, microbial respiration depletes oxygen and drops pH at the contact point. Then, as the coral tissue starts breaking down, bacteria produce hydrogen sulfide, which diffuses outward into neighboring healthy tissue and accelerates the spread of death across the colony.
This is why you sometimes see a patch of dead tissue on the side of a coral facing a dead spot in your flow pattern, while the rest of the colony looks fine. The fix is straightforward: make sure every coral in your tank receives enough flow to keep its surface clean of settling particles. Powerheads on a wave-making cycle help prevent any single area from becoming a permanent dead zone.
Rapid Tissue Necrosis
RTN is the nightmare scenario. Your coral looks healthy in the evening, and by morning, tissue is peeling off the skeleton in sheets, leaving bare white calcium carbonate behind. The progression can kill an entire colony in 24 to 48 hours. Research has linked RTN to a bacterial infection, with Vibrio bacteria found in significantly higher concentrations in diseased corals compared to healthy ones. Diseased tissue showed a specific Vibrio strain making up over 15% of the total bacterial community, compared to less than 5% in healthy corals.
RTN typically strikes after a stress event: a temperature spike, a parameter swing, or physical damage that gives bacteria an entry point. Healthy corals with intact tissue and a strong immune response resist infection. Stressed corals with compromised tissue become vulnerable. If you notice RTN starting, the best immediate action is to frag the coral well ahead of the advancing tissue loss line. The healthy fragment can sometimes survive if moved to stable conditions. There’s no reliable chemical treatment once RTN is actively progressing.
Pests You Can’t See
Acropora-eating flatworms are one of the most frustrating coral pests because they’re nearly invisible on the coral in your tank. These tiny flatworms, some as small as 3mm, match the color of the coral they’re feeding on. The first sign is usually the coral gradually losing color, fading day by day for no apparent water chemistry reason. On closer inspection, you may notice small oval bite marks where the flatworms have consumed tissue.
By the time the damage is obvious, only the outer edges of the colony may still have color, with the core severely affected. The best way to detect flatworms is to remove a suspect coral and dip it in a coral pest treatment solution. Flatworms will release from the tissue and become visible as flat oval disks in the dip water. Other common pests include montipora-eating nudibranchs, red bugs on Acropora, and various types of coral-eating snails. Quarantining and dipping every new coral before adding it to your display tank prevents most infestations.
Copper and Chemical Contamination
Copper is lethal to corals and most invertebrates at concentrations so low they’re hard to detect without a quality test kit. Invertebrates should not be exposed to copper above 0.01 mg/L, and ideally the level should be zero. The problem is that copper binds to surfaces in your tank, including rock, substrate, and decorations, and can be released later if pH drops or water chemistry shifts. A tank that was ever treated with copper-based fish medication can leach copper for months or years afterward.
Common sources of accidental copper contamination include uncoated brass fittings in plumbing, copper pipes in your home’s water supply (especially with new plumbing or low-pH tap water), and medications used in a tank or sump that later houses corals. Even feeding brine shrimp that were exposed to copper can introduce enough of the metal to harm sensitive invertebrates. If you suspect contamination, test with a copper test kit that reads below 0.01 mg/L, and run a chemical filtration media designed to remove heavy metals.
Too Much or Too Little Light
Corals depend on light for energy, but the relationship is more nuanced than “more light equals better growth.” Corals adapt to specific light levels, and changing those levels abruptly in either direction causes damage. Research on the coral Stylophora pistillata showed the species can survive light levels ranging from 95% of surface sunlight down to less than 1%, but the coral must acclimate gradually. Under extremely low light, corals lose their symbiotic algae through degradation, essentially digesting them as an energy source of last resort.
In aquariums, the most common lighting mistake is placing a new coral under intense light it wasn’t acclimated to. A coral grown under moderate light at a retailer’s tank and placed directly under a high-output LED fixture will photo-bleach, turning pale or white from the top down. The reverse also happens: corals moved to a dimmer location slowly brown out as they try to compensate by packing more pigment into fewer algae cells. When adding new corals, start them lower in the tank or at reduced light intensity and increase over two to three weeks. When upgrading your light fixture, reduce the intensity to your previous levels first, then ramp up gradually.