True fungal infections in aquarium fish are most often caused by water molds of the genus Saprolegnia and related oomycetes such as Achlya. Despite being commonly called fungi, these organisms are technically oomycetes — water molds that behave similarly to fungi. Saprolegnia produces characteristic cottony, gray-white filamentous growths on the skin, fins, gills, or eggs and is almost always a secondary infection that follows physical injury, parasitic disease, or compromised slime coat. Fungal infections are often confused with columnaris, which looks similar but is bacterial. Correct identification is important because columnaris requires antibiotics while true fungus responds to antifungal treatments.

Overview

Saprolegnia spores are present in virtually all freshwater environments and are opportunistic, only causing disease when the protective slime coat has been damaged. Common triggers include:

  • Physical injury from handling, aggression, or rough decor
  • Prior parasitic infection (especially ich) that damaged the skin
  • Poor water quality, particularly low temperature with high organic load
  • Dead or unfertilized eggs in breeding tanks (a frequent site of Saprolegnia growth)
  • Immunosuppression from stress, nutritional deficiency, or concurrent disease
  • Cool water in tropical species reduces immune response and favors water mold growth

Saprolegnia is particularly common in cold-water systems, outdoor ponds in early spring, and breeding setups where dead eggs serve as a starting point for infection.

Symptoms

  • Gray-white, cottony, fuzzy growths on the skin, fins, or gills
  • Fluffy, cotton-ball appearance that moves in the water current
  • Often localized to sites of previous injury or ich spots
  • Tissue underneath the fungal growth may become ulcerated
  • Loss of appetite and lethargy in advanced cases
  • Eggs turning white and fuzzy in breeding tanks — can spread to healthy eggs if not removed
  • Secondary bacterial infection as tissue is damaged

Fungal growths generally look softer and more three-dimensional than the flatter, harder patches of columnaris.

Diagnosis

Diagnosis is typically visual. Under a microscope, a scraping of the affected area reveals characteristic branching, non-septate hyphae — classic for Saprolegnia and related oomycetes. This distinguishes fungal infection from columnaris bacteria (which show as thin, gliding rods that form columns). For atypical or refractory cases, an aquatic veterinarian can confirm the diagnosis and rule out mixed infections.

Treatment

  • Identify and correct underlying causes — Improve water quality, remove aggressive tankmates, check for concurrent parasites, and address any injuries. Without this, fungal infection will recur.
  • Methylene blue bath — Effective antifungal for fish and eggs. Often used in a hospital tank or as a dip for valuable specimens. Stains silicone and ornaments blue.
  • Pimafix — Mild antifungal from API, suitable for early cases but generally weaker than methylene blue or traditional antifungals
  • Ich-X (formalin and malachite green) — Malachite green has strong antifungal activity and is effective for both fish and egg fungus. Half dose for scaleless species.
  • Aquarium salt — At 1 teaspoon per gallon, salt inhibits Saprolegnia growth in salt-tolerant species and is often used prophylactically in breeding setups
  • Physical removal — For localized growths on strong fish, gently swab with a cotton swab soaked in dilute antifungal during a brief out-of-water handling
  • Remove affected eggs — In breeding setups, remove white, fuzzy eggs daily to prevent spread to healthy eggs

Maintain excellent oxygenation during treatment — antifungals and high organic loads reduce dissolved oxygen. Remove activated carbon.

Prevention

  • Maintain excellent water quality and stable temperature
  • Reduce injury risk — smooth decor, compatible tankmates, gentle handling
  • Quarantine and promptly treat any parasitic disease
  • Use aquarium salt prophylactically in breeding setups where appropriate
  • Remove dead eggs promptly in breeding tanks
  • Ensure adequate nutrition to support immune function
  • Keep pond fish protected through winter transitions when immunity is low

When to Consult a Veterinarian

Persistent, widespread, or rapidly progressing fungal infection warrants veterinary evaluation. Fungal-like growths that do not respond to antifungal treatment may actually be columnaris or a mixed infection, and an aquatic veterinarian can confirm diagnosis and recommend appropriate treatment.