Columnaris (Mouth Fungus): How to Treat This Fast-Killing Bacterial Disease in Aquarium Fish

You saw white cottony stuff around your fish’s mouth. Maybe gray patches near the dorsal fin. The fish seems lethargic. Within hours, another fish shows the same signs.

That is Columnaris—and it is not a fungus despite the nickname “mouth fungus.” It is a bacterial infection that spreads fast enough to wipe out an entire tank in a single day.

What Columnaris Is

Columnaris is caused by Flavobacterium columnare (formerly called Flexibacter columnaris). It is a gram-negative bacterial infection that affects freshwater fish.

The white cottony patches look fungal, which is why many beginners misdiagnose it. But Columnaris bacteria produce enzymes that degrade fish tissue, creating the fuzzy appearance.

Multiple names refer to the same disease:

• Mouth Fungus
• Cotton-Wool Disease
• Saddleback Disease
• Flexibacter (older name)

Recognizing the Symptoms

Look for these presentations:

Mouth Form

• Gray or white line around the lips
• White tufts sprouting from the mouth area like cotton
• Mouth may appear eroded or rotting

Saddleback Form

• Gray patch near the dorsal fin
• Pale band wrapping around the body like a saddle
• Often accompanied by skin erosion

Fin Form

• Fin edges turn white or gray
• Fins appear ragged, eroding from the tips
• May progress to fin rot appearance

Additional signs:

• Fish becomes lethargic
• Rapid breathing, gasping at surface
• Yellowish-brown centers in ulcers (from enzyme activity)
• Fish may stop eating

Why It Spreads So Fast

Columnaris enters through gills, mouth, or small wounds. Once inside, the bacteria multiply rapidly and produce tissue-degrading enzymes.

The disease can run in two patterns:

• Chronic form: Progresses over days or weeks
• Acute form: Kills fish within hours, can eliminate whole tank populations overnight

The acute form is what scares experienced aquarists. A tank can look normal in the morning and have multiple dead fish by evening.

Contaminated equipment spreads Columnaris between tanks. Nets, containers, even shared food can carry the bacteria.

Who Gets It

Livebearers—guppies, mollies, platies—are particularly susceptible. Catfish also show high vulnerability.

But any freshwater fish can develop Columnaris when stressed.

Trigger Factors

Columnaris outbreaks follow stress. Common triggers:

• Poor water quality (ammonia, nitrite)
• Unstable pH
• Overcrowding
• Inadequate diet
• Low oxygen
• Recent handling or transport
• Temperature stress

The bacteria may be present in many tanks without causing disease. When fish become stressed, their immune defenses drop, and Columnaris takes hold.

Treatment Protocol

Speed is critical. Start treatment immediately when you see signs.

Step 1: Identify Correctly

Confirm it is Columnaris, not true fungus. Fungal infections show white cottony growth but typically appear as distinct tufts on wounds, not the eroding mouth or saddleback pattern of Columnaris.

If unsure, treat for Columnaris anyway—the acute form does not wait for perfect diagnosis.

Step 2: Choose Antibiotics

Columnaris is a gram-negative bacterial infection. Use gram-negative antibiotics:

Strong options:

• Erythromycin (Maracyn): Effective against Columnaris
• Minocycline (Maracyn II): Treats Aeromonas, common secondary infection
• Kanamycin (Kanacyn): Covers both gram-positive and gram-negative

Recommended combination: Use Maracyn + Maracyn II together. This covers Columnaris and the Aeromonas bacteria that often accompany it.

Other options:

• Penicillin: 10,000 units per liter, second dose in two days
• Chloromycetin: 10-20 mg per liter, second dose in two days
• Tetracycline or Oxytetracycline
• Copper sulfate
• Furan
• Potassium permanganate (cautious use—can harm fish at wrong doses)
• Nifurpirinol
• Acriflavine
• Malachite green

For internal treatment, medicated food with Terramycin (oxytetracycline) works when fish are still eating.

Step 3: Dose Correctly

Follow package instructions for commercial products. For measured antibiotics:

• Penicillin: 10,000 units per liter, repeat in two days
• Chloromycetin: 10-20 mg per liter, repeat in two days

Treat for the full recommended duration. Some Columnaris strains show antibiotic resistance—stopping early allows survivors to multiply.

Step 4: Improve Water Quality

Antibiotics alone may fail if the underlying stress continues. Simultaneously:

• Test ammonia and nitrite
• Do a partial water change
• Ensure adequate aeration
• Reduce crowding if possible

Some antibiotics can harm beneficial bacteria in your filter. Monitor ammonia during treatment and use an ammonia remover if needed.

Step 5: Disinfect Equipment

Any net, container, or tool used in the infected tank can carry Columnaris to other tanks. Disinfect with bleach solution or boiling water before using elsewhere.

Common Mistakes

Treating as Fungus Only

Antifungal medications do not kill Columnaris bacteria. Using fungus treatments delays effective care.

If you see cottony mouth patches, start gram-negative antibiotics immediately.

Stopping Treatment Early

Some fish look better after 2-3 days. The bacteria may still be present. Complete the full antibiotic course.

Ignoring Secondary Infections

Aeromonas often accompanies Columnaris. Using only single-antibiotic treatment may miss the secondary bacteria.

Combination treatment (Maracyn + Maracyn II) covers both.

Forgetting About Filter Bacteria

Some antibiotics disrupt biological filtration. Watch for ammonia spikes during treatment. Have ammonia remover ready.

Prevention

Columnaris arrives through stressed fish. Prevention focuses on reducing stress:

• Maintain stable water: Zero ammonia, zero nitrite, consistent pH
• Regular water changes: Weekly partial changes keep conditions stable
• Good biological filtration: Adequate filter capacity for your tank size
• Varied diet: Nutrition supports immune function
• Proper aeration: Sufficient oxygen prevents stress
• Quarantine new fish: 2-4 weeks observation before adding to main tank
• Disinfect shared equipment: Nets and containers can carry bacteria between tanks

Columnaris rarely appears in well-maintained tanks with unstressed fish. The bacteria need a compromised host.

Summary

Columnaris is a fast-moving bacterial infection that looks like fungus but requires gram-negative antibiotic treatment. Recognize the signs: cottony mouth patches, saddleback lesions near the dorsal fin, eroding fins.

Treat immediately with erythromycin (Maracyn), kanamycin (Kanacyn), or penicillin. Use Maracyn + Maracyn II together to cover Columnaris and Aeromonas.

Speed matters. Acute Columnaris can kill whole tank populations in hours. Do not wait for perfect diagnosis—start antibiotics when you see suspicious signs.

Prevent outbreaks by maintaining excellent water quality and reducing stress. Columnaris exploits weak fish in poor conditions.