White Fungus on Fish

"White fungus on fish" is one of the most common disease-search queries in the hobby, and the misdiagnosis rate is staggering. Most aquarists see a white fuzzy patch, assume it's fungal, reach for an antifungal medication, and wonder why the fish dies anyway. That's because what looks like "white fungus" is often columnaris — a bacterial infection that laughs at antifungal treatment. Getting the diagnosis right is the difference between saving and losing the fish.

I lost a beautiful group of harlequin rasboras to columnaris before I learned the difference. The white patches around their mouths looked exactly like the Saprolegnia fungus I'd treated successfully before. Wrong diagnosis, wrong treatment, lost fish. This guide is so you don't repeat that mistake.

Symptom Identification: Fungal or Bacterial?

The single most important skill here is distinguishing true fungus (Saprolegnia) from bacterial infection (columnaris). They look similar but respond to completely different treatments:

True Fungus (Saprolegnia) — "Cotton Wool Disease"

• Looks like cotton wool or white fluff — three-dimensional, fuzzy, with strands radiating outward
• Grows on a wound, injury, or damaged area — fungus is opportunistic, infecting broken skin
• Often appears after physical injury — from fighting, netting, or sharp decorations
• Can grow on unfertilized eggs — very common in breeding tanks
• Slow-growing — develops over days, not hours

Bacterial (Columnaris) — "Mouth Fungus"

• Flat white or grey patches — mat-like, not fluffy, lying against the skin
• Often around the mouth, head, or as a saddle-back lesion across the dorsal
• Rapid progression — spreads in hours, not days
• Often in warm water (26°C+) — thrives in warmth, opposite of fungus
• Frayed fins with white edges often accompany mouth patches

The diagnostic test: look at the patch from the side. If it stands out from the body like a tuft of cotton, it's fungal. If it lies flat against the skin like a coating, it's bacterial columnaris. When in doubt, treat for both — but understand you're treating two different things.

Cause Diagnosis

Saprolegnia (True Fungus)

Saprolegnia is a water mould (not technically a fungus, but treated like one). It's present in every aquarium but only infects fish with compromised skin — from injuries, ammonia burns, parasite damage, or stress. Common triggers: physical injury from netting or rough handling, fin-nipping tankmates, ammonia spikes that damage the slime coat, or unfertilized eggs left in the tank. The mould feeds on dead tissue and spreads outward.

Columnaris (Bacterial)

Columnaris is caused by Flavobacterium columnare, a bacterium that thrives in warm, slow-moving water with high organic load. It enters through gills and wounds, and spreads rapidly. Triggers: high bioload, warm temperatures (especially above 28°C), poor water quality, stress from transport, and introduction of infected fish. Highly contagious — one sick fish can infect a whole tank within hours.

Treatment Protocol

For True Fungus (Saprolegnia)

1. 1
   Do a 30% water change and test parameters

   Clean water is the foundation. Ammonia burns feed fungus — fix water quality first.

2. 2
   Move fish to a hospital tank

   Fungal treatments stain everything and kill biofilters. A 5–10 gallon hospital tank with a sponge filter is ideal.

3. 3
   Add aquarium salt at 1 tablespoon per 5 gallons

   Salt is highly effective against Saprolegnia. Maintain concentration through water changes.

4. 4
   Treat with methylene blue or antifungal medication

   Methylene blue at 1 drop per gallon, or commercial antifungals like API Fungus Cure or Pimafix (mild cases only). Remove carbon from filter.

5. 5
   For severe cases: salt bath

   1 tablespoon salt per gallon of tank water, 15–30 minutes daily. Watch for distress and return to tank if fish rolls or gasps.

6. 6
   Treat for 7–10 days after fungus disappears

   Spores persist — stopping early leads to recurrence.

For Columnaris (Bacterial)

1. 1
   Lower tank temperature to 22–24°C

   Columnaris thrives in warmth. Cooling the tank slows the bacteria dramatically — if species allow. Don't go below 20°C for tropicals.

2. 2
   Move to hospital tank and treat with antibiotics

   Kanaplex (kanamycin) or API Furan-2 are the most effective. Columnaris is gram-negative; use antibiotics that target gram-negative bacteria. Follow package dosing exactly.

3. 3
   Increase aeration significantly

   Columnaris damages gills — fish need maximum oxygen. Add air stones, increase surface agitation.

4. 4
   Add aquarium salt at 1 tablespoon per 5 gallons

   Salt reduces bacterial load and supports fish slime coat. Helps with both fungus and columnaris.

5. 5
   Treat the main tank too

   Columnaris is highly contagious. Even if you isolate the visible case, treat the display tank with the same antibiotic protocol to prevent spread.

6. 6
   Continue full antibiotic course — 7–14 days

   Stopping early creates antibiotic-resistant bacteria. Run the full course even if fish look better in 3 days.

Prevention

• Maintain pristine water quality — ammonia and nitrite damage slime coat, opening the door to both fungus and columnaris.
• Remove unfertilized eggs promptly — they grow Saprolegnia within 24 hours and spread it to live fish.
• Avoid injuries — use soft nets, remove aggressive tankmates, smooth sharp decoration edges.
• Quarantine new fish for 4 weeks — columnaris often arrives on new fish and erupts under transport stress.
• Avoid overstocking in warm tanks — warm + crowded + high organics = columnaris outbreak waiting.
• Treat wounds promptly — a fish that's been injured should get clean water and salt before infection sets in.

Common Mistakes to Avoid

White patch misdiagnosis kills more fish than the infections themselves. Avoid these errors:

• Assuming all white patches are fungal — the #1 mistake. Columnaris bacteria look similar to Saprolegnia fungus but require completely different treatment. Diagnose before medicating.
• Using methylene blue in the main tank — it kills biofilters and stains everything blue permanently. Always use a hospital tank for methylene blue treatment.
• Skipping the salt step — aquarium salt is highly effective against both fungus and columnaris. It's cheap, safe for most fish, and supports any other treatment you're running.
• Not lowering temperature for columnaris — columnaris thrives in warm water. Cooling the tank to 22–24°C slows the bacteria dramatically. The opposite of fungal treatment, where warmth sometimes helps.
• Stopping antibiotic treatment early — columnaris that survives partial treatment becomes antibiotic-resistant. Run the full course even if fish look better in 3 days.
• Using Pimafix or Melafix alone for severe cases — these herbal remedies are too weak for established infections. They may help with mild fungus but are useless against columnaris.
• Not disinfecting equipment between tanks — columnaris spreads on nets, hoses, and even wet hands. Bleach anything that touched an infected tank (10% solution, rinse well).
• Treating only the obviously sick fish — both Saprolegnia and columnaris spread through water. Treat the whole tank, not just visible cases — the others are likely already infected.

The takeaway: white patches demand diagnosis before treatment. A few minutes distinguishing cotton-wool fungus (3D, on wounds) from mat-like columnaris (flat, around mouth, spreads fast) determines whether methylene blue or Kanaplex is the right tool. Wrong choice = lost fish.

Quick Diagnosis Table

Frequently Asked Questions