Describe what you're seeing and get a differential diagnosis with treatment recommendations. Select all symptoms that apply - results update after clicking Check Symptoms.
Disease is one of the leading causes of economic loss in aquaculture and one of the most frustrating challenges in hobby fish keeping. Fish, like all animals, carry a baseline pathogen load - bacteria, parasites, and fungi are present in virtually every aquatic environment. Whether disease develops depends on a three-way interaction known as the disease triangle: the pathogen's virulence, the host fish's immune status, and the environmental conditions. Improving any one of these three factors can prevent or resolve disease without medication.
Protozoan parasites such as Ichthyophthirius multifiliis (Ich) and Oodinium (Velvet) are among the most common killers in aquaria. These obligate parasites cycle between a free-swimming infective stage and a feeding stage attached to the fish. Treatment must address the free-swimming stage and span the entire life cycle - typically 2 weeks. Metazoan parasites such as anchor worm (Lernaea) and fish lice (Argulus) are visible to the naked eye and treated mechanically and chemically.
Bacterial infections are almost universally secondary - they exploit fish already weakened by stress, injury, or poor water quality. Aeromonas and Pseudomonas species cause fin rot, ulcers, and dropsy. Flavobacterium columnare causes Columnaris, which presents as white saddle-shaped lesions and is often mistaken for a fungal infection. Antibiotic treatment should be used only when clearly indicated, as resistance is a growing concern in aquaculture.
Saprolegnia (water mould) is the most common fungal pathogen, appearing as white or grey cotton-like tufts on wounds, eggs, or immunocompromised fish. It is almost always secondary to injury or stress. Treatment with salt baths, potassium permanganate, or malachite green is generally effective. True primary fungal infections are rare in fish.
Viral diseases such as Koi Herpesvirus (KHV), Spring Viraemia of Carp (SVC), and Infectious Salmon Anaemia (ISA) have no cure and cause catastrophic losses in aquaculture. Prevention through biosecurity, vaccination (where available), and quarantine is the only effective strategy. Notifiable viral diseases must be reported to veterinary or regulatory authorities.
Quarantine is the single most effective disease prevention measure available to fish keepers and aquaculture producers alike. New fish should be held in a separate, dedicated quarantine system for a minimum of 4 weeks - 6 weeks for sensitive or high-value species. During this period, observe for disease signs, test for key pathogens if warranted, and ensure the fish are feeding and behaving normally before introduction to the main system. The cost of a quarantine tank is trivial compared to the cost of treating - or losing - an entire stock.
Commercial aquaculture operations implement biosecurity protocols analogous to those used in poultry and livestock farming. Key principles include: all-in, all-out stocking to break pathogen cycles; disinfection of equipment, vehicles, and clothing between units; source verification - purchasing stock only from certified disease-free suppliers; water source management including UV sterilisation and filtration; and fallowing - resting pond or sea pen sites between production cycles to reduce pathogen loads in sediment. These measures dramatically reduce reliance on therapeutants and antibiotics.
This tool provides differential diagnoses based on symptom matching and is intended as an educational first-step resource, not a replacement for professional veterinary diagnosis. Many fish diseases share symptoms (white patches can indicate Ich, Columnaris, fungal infection, or velvet depending on texture and context), and accurate diagnosis often requires microscopy, bacterial culture, or PCR testing. When in doubt - particularly in commercial settings or when dealing with notifiable diseases - consult a qualified aquatic veterinarian. Early professional intervention almost always produces better outcomes than self-treatment delays.
Before reaching for medication, always address water quality. The vast majority of disease outbreaks are precipitated by a deterioration in water parameters - high ammonia, low oxygen, pH crash, or temperature stress. Perform a water change, check all parameters, and identify the environmental trigger. If medication is required, follow these principles: remove activated carbon (it adsorbs most therapeutants); treat the entire system (not just the sick fish); complete the full treatment course; and monitor closely for secondary infections during recovery. Document all disease events, treatments, and outcomes - this record becomes invaluable for identifying patterns and preventing recurrence.