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bacterial fungal

Columnaris (cotton mouth / saddleback disease)

Pale grey-white patches on the mouth, body, or dorsal surface often with a saddle-shaped lesion behind the dorsal fin. Caused by Flavobacterium columnare, a gram-negative bacterium. Can progress very rapidly, especially at warmer temperatures.

High prioritypattern match not diagnosis2 source notes

Do first

  • Test water immediately correct any ammonia or nitrite issues.
  • Do NOT raise temperature Columnaris progresses faster in warm water.
  • Remove affected fish to a hospital tank if possible.
  • Increase aeration and water flow.
  • Reduce feeding or stop entirely for 24–48 hours.

Escalate if

  • Rapid progression lesions expanding within hours.
  • Lesions spreading to gills (visible external gill damage).
  • Multiple fish dying within 24–48 hours.
  • No improvement after 3–4 days of correct antibiotic treatment.

Water clues

These readings can push this pattern higher or lower in the triage result.

ammonia above zero+4

Ammonia stress dramatically increases susceptibility to Flavobacterium poor water quality is the most common predisposing factor.

nitrite above zero+3

Nitrite-stressed fish have impaired immunity and are highly vulnerable.

temp above species max+4

Columnaris bacteria thrive and spread much faster at high temperatures warmth accelerates disease progression.

nitrate above 40+2

Chronically high nitrate contributes to immune suppression.

Care protocol

Follow only the steps that fit your species, tank inhabitants, and medication label.

Identifying Columnaris vs. fungal infection

  1. Columnaris lesions are typically grey-white or yellowish, with a dry or slightly rough appearance not as fluffy as true fungal infections.
  2. Look for the characteristic 'saddleback' pattern a pale lesion starting behind the dorsal fin that wraps toward the sides.
  3. Mouth rot appears as whitish erosion or necrosis at the lips or jaw.
  4. Columnaris can look very similar to fungal infection when in doubt, treat with an antibacterial first (or both if urgency is high).
Cautions
  • Columnaris spreads rapidly. Do not wait for perfect identification before acting.
  • Do not confuse with lymphocystis (wart-like nodules) or ich.

Treatment protocol

  1. Treat with a broad-spectrum antibacterial or antibiotic effective against gram-negative bacteria (e.g., kanamycin, nitrofurazone, or doxycycline where available).
  2. If treating in the display tank, remove activated carbon first.
  3. Treat for the full recommended course (usually 5–10 days) do not stop early.
  4. Perform partial water changes between doses as directed by the medication.
  5. Salt baths (short duration) may help as an adjunct treatment for surface lesions.
  6. After treatment, improve water quality maintenance to prevent recurrence.
Cautions
  • Columnaris can become antibiotic-resistant avoid prophylactic antibiotic use.
  • Do not combine with other medications without checking compatibility.
  • Scaleless fish may react poorly to some antibacterial compounds research species first.

Source notes

References and context notes used for this triage entry.

Flavobacterium columnare UF/IFASUniversity of Florida EDIS conservative extension reference.Merck Vet Manual Bacterial Diseases of FishVeterinary reference for Flavobacterium infections.
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Related patterns

Similar symptoms or risk factors worth comparing before treatment.

Medium priorityFin rot or tail damage

Progressive erosion or fraying of fins and tail tissue, often starting at the edges. Can be bacterial (Aeromonas, Pseudomonas, Flexibacter) or fungal, and is strongly associated with water quality issues and physical injury.

UrgentUronema marinum (marine ciliate)

A scavenger ciliate that becomes pathogenic under poor water quality or stress, causing rapidly spreading open lesions, internal hemorrhaging, and fast death in marine fish. Fish appear to melt. Uronema is most dangerous to thin-bodied fish (chromis, small wrasses, clownfish) and is often introduced with uncleaned live rock or wild-caught fish. Notoriously difficult to treat.

Medium priorityFungal infection (Saprolegnia / Achyla)

White, grey, or brownish cottony or woolly tufts growing on the skin, fins, or around the eyes. Saprolegnia and Achyla are water molds (oomycetes) that almost always infect already damaged tissue. They are secondary invaders following injury, bacteria, or parasites.

UrgentBacterial septicemia (hemorrhagic septicemia)

Red streaking, hemorrhaging under the skin, and systemic illness caused by gram-negative bacteria (most commonly Aeromonas hydrophila) entering the bloodstream. Can follow injury, fin rot, poor water quality, or be primary in severely stressed fish. A systemic emergency.