A koi acting unwell may have an environmental problem, an injury, an infection, or several problems together. Start by separating observations from explanations. Gasping, flashing, or an ulcer is a finding; it is not automatically a diagnosis or the name of a medicine.
Use Three Questions First
- How urgent is the situation? Persistent gasping, loss of balance, rapid decline, or several deaths needs prompt help.
- What changed? Check equipment, refills, storms, chemicals, feeding, and fish additions.
- What can be measured? Record temperature, ammonia, nitrite, pH, alkalinity, and oxygen where available.
Understand the Main Categories
CategoryExamples of concernsUseful next informationEnvironmentalLow oxygen, ammonia, nitrite, contaminationEquipment check, water and source-water resultsExternal parasitesSkin or gill irritationAppropriate microscopy and interpretationBacterial or other infectionsUlcers, damaged tissue, systemic illnessExamination and suitable diagnostic testsViral diseaseA concerning outbreak patternHistory and specialist laboratory testingInjury or other conditionsA wound, swelling, or abnormal buoyancyIndividual clinical assessmentThe categories can overlap. Correcting water does not prove that an infection is absent, and finding a parasite does not make water management irrelevant.
Choose the Right Detail Guide
Use the disease and parasite comparison guide to see why signs overlap. KHV and SVC require specialist investigation. An ulcer deserves assessment of both the wound and its possible causes.
Prepare for Professional Help
Send a timeline, fish and system details, photographs or a short video, test values with units, and every recent treatment. Ask what samples are needed before collecting them. Avoid combining treatments while still trying to identify the problem; an undocumented intervention can complicate the next decision.