FRESHWATER disease

Bacterial gill disease (Flavobacterium branchiophilum)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Quick referenceBacterial gill disease (Flavobacterium branchiophilum) is a common freshwater bacterial infection of fish gills. Causes rapid breathing + flared gills; treat with antibiotics + clean water.

Symptoms

Rapid breathing
Gill flaring
Surface gasping
Refusing food
Lethargy
Excess slime coat

Cause

Flavobacterium branchiophilum bacteria proliferate in poor water quality - high ammonia, high organic load, overcrowded tanks. Common in pond systems + commercial farms.

Treatment options

Kanaplex. Most effective. 7-10 day course in QT, daily water changes.
Furan-2. Alternative. Effective but slower.
Maracyn 2 (minocycline). Good for mild cases. Less expensive than Kanaplex.
Salt bath. Adjunct: 1 teaspoon per gallon aquarium salt for 7 days, plus medication.
Improve water quality. Foundation. Ammonia 0, nitrite 0, oxygen high (airstone). Reduce stocking 30%.

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.

Prevention

Avoid overstocking. Maintain water quality. Clean filter media regularly. Quarantine new fish.

Fatality + outcome

Moderate - kills 20-40% of stock if untreated. Stressed fish die fastest.

Full disease database · Symptom matcher · Q&A library · Glossary · QT timeline calculator

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.