Most visible koi disease signs have more than one possible cause. Use this comparison to organize observations and find the relevant guide, not to diagnose from a photograph. Check water and life support while obtaining appropriate fish-health help.
Compare the Pattern of Signs
FindingPossible categoriesNext informationRubbing or excess mucusWater irritation; external parasitesWater results and trained microscopic examinationWhite spotsIch or other skin changesDistribution, gill signs, examinationUlcers or rednessInjury, bacterial disease, parasites, other damageProgression and professionally selected samplesSwelling or raised scalesA syndrome with several possible underlying causesHistory and veterinary examinationBreathing trouble or deathsOxygen/water problems, gill disease, infectionUrgent system checks and professional assessmentExternal Parasites Need Identification
Read Ich, Trichodina, Costia, and skin and gill flukes for differences. Argulus and Lernaea have different environmental stages. One salt or medicine recipe does not cover every parasite. Trained examination may be needed even when an attached organism is visible.
Ulcers Do Not Identify a Bacterial Species
The appearance of a wound does not distinguish Aeromonas from Pseudomonas or other causes. Culture and susceptibility testing may guide treatment. Expanding lesions or systemic decline need prompt veterinary help; prescribed antibiotics are not a substitute for correcting underlying conditions.
Investigate Group Illness
Several fish declining warrants checks for environmental failures and infectious disease. KHV and SVC require professional investigation and laboratory interpretation. Pause transfers and avoid sharing wet equipment from a suspect system.
Bring a Timeline, Not a Guess
Record affected fish, onset, temperature, water results, additions, maintenance, and treatments. See urgent response for distress and treatment decisions before choosing a product.