Fungal Infection

When to act urgently

Breathing distress, rapid deterioration, inability to remain upright, major wounds or multiple deaths require urgent aquatic-veterinary or fish-health help. Check aeration and water immediately; do not wait to match a photograph.

What you may notice

• Pale, cotton-like growth may appear on damaged skin or fins.

Possible causes and look-alikes

• A water mold such as Saprolegnia may be involved; water molds are not true fungi. Some bacterial lesions can resemble this appearance.

Check first

• Record species, aquarium type, recent arrivals or changes, appetite, breathing, and how many fish are affected. Check aggression, injuries, filtration and aeration.
• Test ammonia, nitrite, pH and temperature; measure dissolved oxygen where possible and salinity in marine/brackish systems. Correct verified environmental problems using suitably prepared water, avoiding abrupt chemistry changes.
• Look for trauma, decaying material, and unsuitable temperature or water conditions. Record where growth started.

Treatment and correction context

• Spreading lesions or gill involvement need professional sampling. Managing the underlying injury or disease is part of the plan.
• Do not choose an antifungal from texture alone or assume a product is safe for all fish and invertebrates.

Record for your next step

Write down onset and progression, species, affected/total fish, recent introductions, feeding observations, dated water results with kit units, and every action or product already used. Contact the diagnostic service before collecting or shipping samples.

Sources and scope

Source checks: 2026-10-03. Educational editorial summary; no independent veterinary review.

• Merck Veterinary Manual: Parasitic Diseases of Fish

  Parasite groups, identification and organism-specific management.

• UF/IFAS: Introduction to Fish Health Management

  Water quality, husbandry and investigation before disease treatment.

• UF/IFAS: Submission of Fish for Diagnostic Evaluation

  Case history and contacting a diagnostic service before collecting samples.