Disease Guide
Browse condition pages built from TankFlare symptom triage rules. Start with water and oxygen, then use the pattern guides to decide what to inspect, isolate, and escalate.
Water first, diagnosis second
Ammonia, nitrite, oxygen, pH shock, temperature stress, and chlorine can look like disease. Confirm water readings before dosing medication.
bacterial fungal
3 conditions
Pale grey-white patches on the mouth, body, or dorsal surface often with a saddle-shaped lesion behind the dorsal fin. Caused by Flavobacterium columnare, a gram-negative bacterium. Can progress very rapidly, especially at warmer temperatures.
Open guideMediumFin rot or tail damageProgressive erosion or fraying of fins and tail tissue, often starting at the edges. Can be bacterial (Aeromonas, Pseudomonas, Flexibacter) or fungal, and is strongly associated with water quality issues and physical injury.
Open guideMediumFungal infection (Saprolegnia / Achyla)White, grey, or brownish cottony or woolly tufts growing on the skin, fins, or around the eyes. Saprolegnia and Achyla are water molds (oomycetes) that almost always infect already damaged tissue. They are secondary invaders following injury, bacteria, or parasites.
Open guidechemical toxicity
5 conditions
Acute distress or death immediately or within hours of a water change, caused by tap water added without a dechlorinator (or with an insufficient amount). Chlorine and chloramine damage gills and cause chemical burns. Chloramine is more stable and harder to remove than chlorine.
Open guideUrgentCO2 overdose (in planted tanks)Fish gasping at the surface or lying on the bottom in a tank with CO2 injection. Excess CO2 displaces oxygen and acidifies the water simultaneously. Particularly common at lights-off when plants stop consuming CO2, or after a sudden increase in CO2 injection rate.
Open guideUrgentCopper toxicityFish distress, lethargy, loss of balance, or death caused by copper levels in the water either from a copper-based medication overdose, copper pipes in the water supply, or accidental cross-contamination from a treated tank. Copper is effective against parasites but has a very narrow margin between therapeutic and lethal doses.
Open guideUrgentMedication overdose or chemical toxicityFish showing acute distress gasping, listing, rapid gill movement, or death shortly after medication was added to the tank. Can be caused by incorrect dosing, stacking medications, or the presence of sensitive species (scaleless fish, invertebrates, labyrinth fish) that cannot tolerate standard doses.
Open guideHigh prioritySalt overdose / osmotic stress from excess saltFish showing distress after aquarium salt was added to a freshwater tank gasping, darting, mucus production, or sudden death. Caused by adding too much salt too quickly, or by adding salt to tanks with salt-sensitive species (scaleless fish, planted tanks, many soft-water species).
Open guideenvironmental
4 conditions
All or most fish gasping at the surface, especially early in the morning. Caused by insufficient gas exchange poor surface agitation, overstocking, warm temperatures, algae crashes, or power failures. Oxygen depletion can kill an entire tank within hours.
Open guideHigh priorityGas bubble disease (supersaturation)Tiny bubbles visible under the skin, in the eyes, on the fins, or in the gills, caused by water that is supersaturated with dissolved gases (usually nitrogen or oxygen). Fish may gasp at the surface while water tests appear normal. Can be caused by using very cold tap water in a warm tank, deep well water, or a pressurized water supply line.
Open guideHigh priorityStray voltage / electrical leakageFish acting erratically, refusing to enter parts of the tank, or showing persistent stress with no water quality explanation. Caused by a small electrical current leaking into the tank water from faulty equipment. Can be chronic and low-level (causing ongoing stress and immune suppression) or acute and fatal. A GFCI (ground fault circuit interrupter) outlet can detect and prevent this.
Open guideHigh priorityTemperature stress (too hot, too cold, or rapid change)Fish becoming lethargic, gasping, or dying due to temperature outside their tolerance range or a rapid change in temperature. Both extremes are harmful cold suppresses immunity and metabolism, while heat reduces dissolved oxygen and accelerates metabolic demand simultaneously.
Open guideenvironmental behavioral
2 conditions
A fish that is persistently stressed over weeks or months will develop a suppressed immune system, making it vulnerable to infections that healthy fish would fight off. Chronic stress is itself a disease state that produces predictable signs and predisposes fish to virtually every other condition in this catalog. Identifying and removing the stressor is the only effective treatment.
Open guideMonitorStress coloration (stress stripes, blanching, or darkening)Unusual color changes horizontal stress bars, vertical fright stripes, pallor, or patchy darkening that indicate the fish is under significant psychological or physiological stress. Not a disease in itself, but a reliable indicator that something in the environment is wrong. Very common in cichlids, bettas, and many schooling fish.
Open guideexternal parasite
2 conditions
Visible worm-like parasites embedded in the flesh, often with inflamed red attachment sites. Lernaea are actually crustaceans, not true worms. Most common in goldfish, koi, and other pond fish but can affect aquarium species. Highly visible to the naked eye once adult females are attached.
Open guideHigh priorityFish lice (Argulus)Flat, disc-shaped crustacean parasites (1–10 mm) visible on the body or fins. Fish show intense irritation, flashing, and scratching. Argulus use sucking mouthparts to feed on blood and inject a toxin that causes lesions and immune suppression. Most common in ponds but can infect aquarium fish.
Open guideinternal parasite
3 conditions
Thin, reddish-brown worms visibly protruding from the fish's vent (anal opening). Camallanus are internal roundworms that embed in the gut lining. One of the few internal parasites visible to the naked eye. Highly contagious within a tank all fish must be treated, not just visibly affected individuals.
Open guideMediumInternal parasites / wasting diseaseProgressive weight loss and sunken belly despite normal or increased appetite, often with abnormal feces (stringy, white, or mucus-laden). Caused by various internal parasites including Hexamita, Spironucleus, Capillaria, or other nematodes. Common in wild-caught fish and recently imported specimens.
Open guideHigh priorityNeon tetra disease (Pleistophora hyphessobryconis)Progressive color loss (the neon-blue stripe fades or turns white in patches), body deformity, and wasting in small tetras and related species. Caused by the microsporidian parasite Pleistophora. Highly contagious, incurable, and almost always fatal affected fish should be humanely euthanized and removed promptly.
Open guidemetabolic dietary
2 conditions
Mild-to-moderate abdominal swelling without raised scales, often linked to overfeeding, a diet too low in fiber, or dry food that expands after eating. Very common in bettas, goldfish, and other heavy eaters. Distinguished from dropsy by the absence of pinecone scales and by a more localized, often asymmetric swelling.
Open guideMonitorOverfeeding and obesityA fish that is visibly overweight rounded or bulging abdomen that has developed over weeks or months due to consistently feeding too much. Unlike dropsy or bloat from constipation, this is a chronic condition. Common in bettas, goldfish, and cichlids. Fat deposits around organs can cause liver disease, buoyancy problems, and shortened lifespan.
Open guideparasite like
7 conditions
A rapidly progressing skin ciliate that preferentially attacks clownfish (Amphiprion spp.) but can infect other marine fish. Causes excessive mucus, sloughing skin, rapid breathing, and fast deterioration. Often fatal within 24–48 hours without treatment. Can be confused with marine velvet but has a distinct slimy, sloughing appearance.
Open guideHigh priorityGill flukes or body flukes (Dactylogyrus / Gyrodactylus)Rapid breathing, gill flaring, flashing, and scratching caused by microscopic monogenean parasites on the gills (Dactylogyrus) or skin/fins (Gyrodactylus). Often introduced with new fish. Requires microscopic confirmation for certainty but may be treated empirically when symptoms are consistent.
Open guideHigh priorityIch-like white spot patternSalt-grain white spots with irritation and flashing, especially after recent livestock additions or temperature drops. Caused by Ichthyophthirius multifiliis (freshwater) or Cryptocaryon irritans (marine).
Open guideHigh priorityMarine ich (Cryptocaryon irritans white spot disease)The marine equivalent of freshwater ich white salt-grain spots on skin and fins, with flashing and rapid breathing. Caused by the ciliate Cryptocaryon irritans. Nearly ubiquitous in marine aquariums and one of the most common diseases in reef tanks. The display tank must go fallow while fish are treated in a hospital tank, because Cryptocaryon cannot be treated safely in the presence of corals and invertebrates.
Open guideUrgentMarine velvet (Amyloodinium ocellatum)The marine equivalent of freshwater velvet an extremely fast-moving dinoflagellate parasite causing gold or rust-colored dust on marine fish, rapid breathing, and quick death. Amyloodinium is considered one of the most dangerous diseases in the marine aquarium hobby and can kill an entire reef tank's fish population within 2–3 days if untreated.
Open guideUrgentUronema marinum (marine ciliate)A scavenger ciliate that becomes pathogenic under poor water quality or stress, causing rapidly spreading open lesions, internal hemorrhaging, and fast death in marine fish. Fish appear to melt. Uronema is most dangerous to thin-bodied fish (chromis, small wrasses, clownfish) and is often introduced with uncleaned live rock or wild-caught fish. Notoriously difficult to treat.
Open guideUrgentVelvet (Oodinium / Amyloodinium)Fine gold, rust, or yellowish dust-like shimmer on the body and fins, often with intense flashing and rapid breathing. Caused by the dinoflagellate parasite Oodinium (freshwater) or Amyloodinium (marine). Often missed until advanced because the coating is subtle.
Open guidesystemic bacterial
3 conditions
Red streaking, hemorrhaging under the skin, and systemic illness caused by gram-negative bacteria (most commonly Aeromonas hydrophila) entering the bloodstream. Can follow injury, fin rot, poor water quality, or be primary in severely stressed fish. A systemic emergency.
Open guideUrgentDropsy (pinecone scales / severe bloating)Severe abdominal bloating with scales raised outward like a pinecone, indicating fluid accumulation in the body cavity. Dropsy is a symptom complex not a single disease caused by internal bacterial infection, organ failure, or severe parasitic infestation. Prognosis is often poor by the time scales are raised.
Open guideHigh priorityFish tuberculosis (Mycobacterium)Chronic, progressive wasting with a combination of sunken belly, spinal deformity (bent spine), skin ulcers, and color loss. Caused by Mycobacterium marinum or related species. Incurable, highly persistent in tank environments, and importantly can cause skin infections in humans. Handle affected fish and their tank water with care.
Open guidesystemic internal
2 conditions
A section of intestine protruding from the vent, appearing as a pink, red, or brownish tube-like structure. Often confused with Camallanus worms, but prolapsed tissue is wider, irregular, and does not move independently. Usually caused by severe constipation, straining, internal parasites, or bacterial infection of the gut.
Open guideMediumSwim bladder disorderLoss of buoyancy control fish floats at the surface, sinks to the bottom, swims sideways, or struggles to maintain position. Can be caused by bacterial infection, constipation/overfeeding, physical injury, genetic defect (common in fancy goldfish), or internal parasites.
Open guideviral
4 conditions
Carp pox (Cyprinid herpesvirus 1) causes smooth, waxy, raised lesions on koi and goldfish, appearing as whitish or pinkish blobs on the skin and fins. Epitheliocystis causes white cyst-like raised spots on the gills and body, caused by intracellular bacteria (Chlamydiales). Both are generally non-lethal in well-managed fish but indicate a stressed or immune-suppressed animal.
Open guideUrgentKoi herpesvirus (KHV / Cyprinid herpesvirus 3)A highly contagious and often fatal viral disease affecting koi (Cyprinus carpio) and common carp. Causes gill damage, lethargy, sunken eyes, and mass mortality typically at water temperatures between 16–25°C (60–77°F). A notifiable disease in many countries. There is no cure; management focuses on biosecurity and welfare decisions.
Open guideMonitorLymphocystis (viral wart-like growths)Rough, cauliflower-like or wart-like nodules on fins or body, caused by Lymphocystivirus. A chronic viral condition with no curative treatment, but. It is rarely fatal. Fish remain infectious. Nodules may regress on their own with improved water quality and reduced stress.
Open guideUrgentSpring viremia of carp (SVC / Rhabdovirus carpio)A notifiable viral hemorrhagic disease affecting cyprinids (koi, goldfish, carp) primarily in spring at water temperatures of 11–17°C (52–63°F). Causes hemorrhaging, abdominal swelling, protruding eyes, and mass mortality. SVC is a regulated disease in many countries suspected outbreaks must be reported to animal health authorities.
Open guidewater quality
4 conditions
Gasping, gill damage, and lethargy caused by elevated ammonia (NH3) or nitrite (NO2) most common in new tanks, after filter failures, or following sudden bioload increases.
Open guideMonitorChronic nitrate stressGradual decline in fish health, color, appetite, and immunity caused by persistently elevated nitrate (NO3). Often overlooked because symptoms develop slowly and fish can survive high nitrate for extended periods before showing clear signs. Particularly damaging for sensitive species.
Open guideHigh priorityNew tank syndrome (uncycled tank)Recurring or persistent fish illness in a tank less than 6–8 weeks old, almost always caused by the absence of an established nitrogen cycle. Beneficial bacteria that convert ammonia to nitrite and then to nitrate have not yet colonized the filter. Fish suffer from toxic ammonia and nitrite spikes.
Open guideUrgentpH shock or rapid pH instabilitySudden fish distress, gasping, excessive mucus, or death following a rapid change in pH. Even a shift of 0.5 pH units within a few hours can be harmful; a shift of 1.0 or more is often fatal. Can occur after large water changes, adding CO2, acid treatments, or alkalinity crashes.
Open guide