Problems with corals and sessile inverts are notoriously hard to diagnose…


Here’s my quick guide of how to diagnose symptoms of ill health and correct the problem.

Fish usually display obvious symptoms specific to a particular problem. But sessile invertebrates (ones attached to the substrate for example corals and anemones) most often do not!

As you probably know, this can make correcting problems quite difficult.

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Most coral and other invertebrate symptoms are simply a result of stress usually caused by:

  • poor and/or fluctuating quality water
  • incorrect lighting

this is the case most of the time.

fixing coral problems

Pretty, but hard to know what is wrong! (credit: dizzylizzy1227)


Yes, there are some pretty nasty coral pests, predators and diseases out there that you need to be aware of…but:

…most of these will not often show up in an aquarium where the new corals have been acclimated correctly and then have been quarantined for 30 days before placing them into the display tank.


Stress is the number one killer of corals and other invertebrates

These are the major forms of stress (in order of likelihood that these are affecting your invertebrate)

  1. Poor/fluctuating water quality – usually temperature and pH, phosphates, nitrates induced stress
  2. Incorrect or inappropriate lighting – bulbs too old, too new, too close, too far away
  3. Not enough water movement
  4. Incompatible tank mates – caused by predation, chemical turf wars, other physical damage
  5. Starvation
  6. Physical damage – caused by collection, handling, pests, parasites, predation
How to fix your sick coral

Usually stress is the cause of coral problems (credit: Stéphane Duquesne)

Stress actually causes these major symptoms in corals and other sessile invertebrates

  1. Tissue recession (or degeneration).
  2. Slowing or stopping of growth.
  3. Coral Alleopathy (chemical warfare between invertebrates).
  4. Lack of polyp expansion in corals, collapsing of anemones.
  5. Bleaching of tissue in corals/photosynthetic anemones.

All these symptoms can be fully recovered from if the stressor is identified (start by testing the water quality!) then kept to a minimum.

Recovery will be sped up if the affected invertebrate is well fed.

curing sick anemones

Your Clownfish will not be pleased if something happens to it’s anemone (credit: Richard Minick)

Lets look at some major symptoms of invertebrate ailment and what is the likely cause…


Symptom 1: Mantle irritations and damage.

Affects: Giant Clams

Probable Diagnosis: Parasitic snails in the family Pyramidellidae (Pyramid snails)

Treatments: Addition of Neon Wrasse; feeds on snails and are colourful and harmless to other species.

diagnosing coral illness

Tridacnid clam mantle damage is usually the result of parasitic snails

Symptom 2: Soft tissue deterioration/shrinking and changing color.

Affects: Corals, zooxanthellae containing clams and anemones.

Probable Diagnosis: If combined with stopping of growth: Too little light or incorrect spectrum bulbs used.

Treatments: Fit bulbs with correct spectrum and wattage, move specimen closer to light source.


Symptom 3: Rapid tissue sloughing from the base up leaving white skeleton resulting in death.

Affects: Corals, most commonly Acroporid species and Pocilloporid corals.

Probable Diagnosis: Rapid Tissue Necrosis (RTN), usually fatal and highly contagious resulting from non-pathogenic stress.

Treatments: No treatment as death is sudden. Can be prevented by quarantining new corals after slow acclimation. Lugols dips, chloramphenicol and fragging unaffected tips can be effective to save colony.

Coral bleaching

Rapid tissue necrosis is a horrible fate for Acropora corals


Symptom 4: Coral tissues appearing white or abnormally light in color

Affects: All corals.

Probable Diagnosis: Bleaching: (mass expulsion of zooanthellae) usually because of physical, chemical, lighting, salinity, diet, poor water movement stresses

Treatments: Remove stress: major cause is temperature, so adjust if necessary. Check all causative stress factors. Coral can recover over time if stress is minimal and they are well fed.


Symptom 5: Progressive area of obvious tissue absence; a white band moving slowly across coral, tissue peeling off in small balls leaving skeleton behind.

Affects: All corals.

Probable Diagnosis: White Band Disease (WBD).

Treatments: Antiseptic bath Lugols solution can help, fragging ahead of band followed by optimal water and lighting conditions.

Curing sick corals

At the first signs of coral illness you should always checking water quality and lighting.


Symptom 6: Black region on coral underneath which tissue is being eaten – Progresses as black band over coral or rotting area on anemones

Affects: All corals, except alcyonarians and fungids. Anemones.

Probable Diagnosis: Anemones: bacterial infection. Coral; Black Band Disease (BBD) caused by cyanobacteria.

Treatments: Antibiotic paste painted onto specimen (e.g. neomycin sulphate), coral: 2-3 days of darkness, fragging ahead of infection.


Symptom 7: Species appearing lethargic and generally unhealthy, possibly not feeding.

Affects: All invertebrates.

Probable Diagnosis: Cytotoxicity caused by copper (i.e. parasite treatment such as product for marine velvet or saltwater Ich).

Treatments: Replace all water in tank immediately, remove afflicted to quarantine.


how to prevent coral disease

Black band disease, you need to act fast! (credit: notesfromtheroad.com)


Symptom 8: Sediment accumulation on coral/sessile Invertebrate.

Could be sand or food/detritus. (can cause bacterial infections).

Affects: All species of sessile invertebrates.

Probable Diagnosis: Often occurs with excess mucous production, algal growth and bacterial infection. The diagnosis is damage caused by poor water flow not removing sediment and/or substrate sifting species (e.g. Gobies).

Treatments: Place specimen higher up in tank, remove substrate-sifting species, and manually remove sediment with powerhead/syringe.


Symptom 9: Specimen cannot right itself (e.g. collapsed anemone).

Affects: Echinoderms (will be upside down), Anemones.

Probable Diagnosis: Bubbles of air trapped within body caused by being removed from water.

Treatments: Carefully attempt to work out bubble with fingers in tank.


Symptom 10: Damaged soft tissue/polyps, excess mucus production in corals, Small dark worms spotted.

Affects: Mostly corals, sometimes-other soft tissued invertebrates.

Probable Diagnosis: Animal predation due to non-compatible species in tank. Convolutriloba Flatworm infestation (.25 of an inch long, reddish brown in colour, 3 pronged tail).

Treatments: Observe tank to find offending animals and remove with tweezers/forceps. This could include species introduced with live rock/sand. If flatworms: freshwater dip afflicted specimen or add small Butterflyfish.


Problems with corals

Prophylactic dips are the best way to prevent introduction of coral eating flatworms

Symptom 11: Obvious increasing algal growth on coral/hard invertebrates.

This symptom can coincide with disease.

Affects: Corals and other hard sessile invertebrates.

Probable Diagnosis: Algae colonisation on weakened or damaged specimens.

Treatments: Add algae eating fish/snails. Reduce light (if not absolutely necessary for corals), increase regular partial water changes to reduce nutrients in water.


Yeah, I know it was long, but hopeful you found this blog post useful. I have actually printed this out and keep this information close to the tank for several years now,  it’s a great quick reference to diagnose and solve problems quickly. Hopefully it will help you too!

If you would like more insiders tips on avoiding mistakes and enjoying more success with your saltwater system, check out my free “Saltwater Success Tools” eBook