Fin Rot

Complete guide to identification, timeline, and treatment

What is Fin Rot?

Fin Rot is a common bacterial (and sometimes fungal) disease affecting both freshwater and saltwater fish. It causes the fins and tail to fray, discolour, and progressively deteriorate. Left untreated, it can reach the body and become life-threatening.

Symptoms

Frayed or ragged fin edges
Discolouration — white, brown, or black fin margins
Fin tissue loss progressing toward the body
Lethargy and loss of appetite
Clamped fins
Red or inflamed fin base in advanced cases

Causes

Poor water quality — high ammonia, nitrite, or nitrate
Bacterial infection (Aeromonas, Pseudomonas, Flavobacterium)
Fungal infection (Saprolegnia) — secondary to bacterial damage
Physical injury from fin-nipping tankmates
Stress and weakened immune system

Fin Rot Life Cycle & Timeline

Week 1
Fin edges begin to look ragged or slightly discoloured. Fish may show mild lethargy. Often mistaken for minor injury at this stage.
Week 2
Fin deterioration becomes more pronounced. Tissue loss progresses inward. Discolouration spreads. Fish appetite declines.
Week 3
Significant fin tissue lost. Infection may reach the fin base. Secondary fungal infection can develop. Fish shows clear signs of stress.
Week 4+
Without treatment, infection reaches the body. Ulceration and systemic infection can develop. Mortality risk is high at this stage.

Treatment Protocol

  • Perform an immediate 30–50% water change and test all parameters — ammonia, nitrite, nitrate, and pH
  • Quarantine the affected fish to reduce stress and allow targeted treatment
  • Treat with an antibiotic effective against gram-negative bacteria
  • Add aquarium salt (1 tsp per 5 gallons) to support healing and reduce osmotic stress
  • Remove any fin-nipping tankmates from the main tank
  • Continue treatment for the full course (7–10 days) and maintain pristine water quality throughout

Medication Options

  • Kanaplex (kanamycin) — highly effective antibiotic for gram-negative bacterial fin rot
  • Furan-2 (nitrofurazone) — broad-spectrum antibiotic; effective for fin rot and body ulcers
  • Maracyn-2 (minocycline) — effective for systemic and surface bacterial infections
  • API Pimafix — antifungal support if secondary fungal infection is present
  • Aquarium salt (1 tsp per 5 gallons) — reduces osmotic stress and inhibits bacterial growth
  • Methylene Blue — mild antifungal and antibacterial; useful for early-stage or mild cases
Important

Fin Rot is almost always caused or worsened by poor water quality. Medication alone will not resolve the issue if the root cause is not addressed. Always improve water conditions first, then treat. Fins can regrow fully if caught early enough.

Important Disclaimer

The information provided in this guide is for educational purposes only. Always consult with a qualified aquatic veterinarian or fish health specialist before administering any treatments. Proceed with treatments at your own risk. We are not responsible for any adverse outcomes.