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Common Aquarium Fish Diseases: Symptoms, Causes and Treatment Guide
2 April 2026
Fish get sick. Even in well-maintained tanks with experienced keepers, disease happens. The difference between losing a fish and saving it almost always comes down to how quickly you spot the problem and how accurately you identify it. This guide covers the diseases you're most likely to encounter, what they look like, what causes them, and what to do.
In this guide
Prevention first — the basics that stop most disease
The vast majority of aquarium fish disease is preventable. This isn't a platitude — it's the practical reality of how fish health works. Fish have immune systems that, when functioning well, resist pathogens that are present at low levels in almost every aquarium. Those immune systems are suppressed by stress. Stress comes from poor water quality, inappropriate parameters, overcrowding, incompatible tankmates, temperature fluctuations, nutritional deficiency, and the trauma of transport.
Address those factors and most fish stay healthy despite the presence of potential pathogens. Neglect them and the same fish succumb to diseases they'd otherwise shrug off. Before reaching for medication, experienced fishkeepers ask one question first: what changed? A tank that develops white spot after a new fish was added without quarantine has a different cause than one that develops it after a heater failure drops the temperature overnight. The answer changes what you do next.
The core preventive practices that eliminate most disease before it starts:
Quarantine all new fish for four to six weeks before introduction to an established tank
Maintain stable water quality — test regularly, change water consistently, don't overstock
Match water parameters to species requirements — fish kept in wrong parameters have chronically suppressed immune function
Feed a varied, high-quality diet — nutritional deficiency is a significant cause of immune suppression
Avoid temperature fluctuations — a stable temperature at either end of the acceptable range is better than a fluctuating one
Don't purchase fish from tanks with visible disease — even healthy-looking fish sharing a tank with sick fish may be carriers
How to diagnose a sick fish
Accurate diagnosis is the most important step in treating a sick fish. Treating for the wrong disease wastes time at best; at worst, it stresses the fish further with inappropriate medication while the actual problem advances.
Observe before you act
Spend five to ten minutes watching the affected fish carefully before doing anything else. Note: what specifically looks wrong (spots, discolouration, fin condition, body shape, lesions), how the fish is behaving (swimming pattern, surface breathing, scratching, hiding), which fish are affected (one, a species, or all), and what has changed recently (new fish, water change, temperature shift, new food).
Test your water first
Before assuming disease, test ammonia, nitrite, nitrate, and pH. A significant proportion of apparent disease presentations are actually water quality responses — fish showing clamped fins, surface breathing, and lethargy due to ammonia or nitrite toxicity rather than infection. Treating a water quality problem with disease medication makes both problems worse. Test first, without exception.
Match symptoms to diagnosis
Use the specific symptom descriptions in this guide to identify what you're dealing with before beginning treatment. Most common diseases have reasonably distinctive presentations. When in doubt, photograph the affected fish and seek a second opinion from an experienced keeper or aquatic veterinarian before medicating.
White spot disease (ich)
Causative agent: Ichthyophthirius multifiliis
What it looks like
Small white spots, approximately 1mm in diameter, scattered across the body, fins, and gills. The spots look like grains of salt — round, raised, and uniform in size. As the infection progresses, spots multiply. Fish typically show behavioural signs before spots become obvious: flicking or flashing (rubbing against surfaces), rapid gill movement, gathering near the surface or filter outlets, and reduced appetite.
What causes it
The Ichthyophthirius multifiliis parasite is present at low levels in most freshwater aquariums. Fish with healthy immune function resist it effectively. Outbreaks occur when immunity is suppressed — most commonly by temperature drops, transport stress, introduction of heavily infected fish, or chronically poor water quality. The parasite has a lifecycle where it feeds on the fish (the visible white spot stage), drops off, encysts on tank surfaces, and multiplies into hundreds of new free-swimming parasites. Understanding this lifecycle is essential to treating it correctly.
Treatment
The parasite is only vulnerable to medication during the free-swimming stage — not while it's protected inside its cyst or feeding on a fish. Treatment must continue long enough to catch all hatching cycles, and raising temperature speeds up the lifecycle to shorten treatment duration.
Remove activated carbon from the filter — carbon strips medication from the water and must come out before dosing
Raise temperature gradually to 28°C — this accelerates the parasite lifecycle, shortening the treatment window. Increase aeration, as warmer water holds less dissolved oxygen
Treat with a proprietary ich medication — formulations containing malachite green, formalin, or acriflavine are standard. Widely available options in the UK include API White Spot Cure, Interpret Anti White Spot, and King British White Spot Control
Repeat doses as directed — typically every 48 hours for the full course. Do not stop early if spots disappear from the fish — encysted parasites may still be present in the tank
Perform a partial water change before each repeat dose
Catfish including plecos, loaches, and scaleless fish can be sensitive to full-dose malachite green treatments — use half the standard dose for these species and monitor closely. Never use copper-based treatments in tanks containing invertebrates — copper is lethal to shrimp and snails at the concentrations required to treat fish disease.
Prognosis
Excellent if caught early and treated correctly. Poor if left untreated — the exponential multiplication means a minor infection becomes severe quickly. Fish that survive heavy ich infections may have lasting gill damage affecting long-term respiratory health.
Fin rot
Causative agents: Aeromonas, Pseudomonas, Flexibacter and other bacteria; occasionally fungal
What it looks like
Progressive deterioration of fin edges — fraying, discolouration (white, brown, or red-tinged edges), and gradual recession of fin tissue. In bacterial fin rot the edges are typically ragged and may have a white or milky border. In fungal fin rot there may be a cotton-wool fuzziness at the affected edges. Advanced cases show significant fin tissue loss — if untreated it can progress to the fin base, at which point full recovery becomes unlikely.
What causes it
Almost always a secondary consequence of something else — poor water quality, physical injury from fin nipping, stress, or a preceding illness that weakened the fish. The bacteria responsible are present in healthy aquariums but only cause disease in fish with compromised immune function or damaged tissue. Fin rot is frequently a symptom that something is wrong with the tank environment rather than a primary disease in its own right.
Treatment
Identify and address the underlying cause first. Test water quality and fix any problems. If fin nipping is occurring, separate the aggressor. Without addressing the cause, fin rot recurs even after successful treatment
Perform a 30–50% water change to immediately improve conditions
Treat with a proprietary antibacterial medication — Interpret Bacterypur, Waterlife Myxazin, or similar. For suspected fungal involvement, use a combined antibacterial/antifungal product
Increase aeration — some antibacterial medications reduce dissolved oxygen
Complete the full treatment course even if improvement is visible
In very early, mild cases in otherwise healthy fish, improved water quality alone may allow natural fin recovery. Monitor closely — if fraying continues to progress at all, medicate.
Prognosis
Good if caught early and the underlying cause is addressed. Fins can and do regrow after fin rot, though severe cases may leave some permanent deformity. Poor if left untreated or if the stressor causing it isn't resolved.
Velvet (Oodinium)
Causative agent: Oodinium pillularis (freshwater)
What it looks like
A fine, dusty, gold or rust-coloured coating on the fish's body and fins — like the fish has been lightly dusted with metallic powder. This distinguishes it from ich, which produces discrete white spots rather than a continuous dusty film. The coating is easiest to see by shining a torch at a low angle across the fish's body in a darkened room. Affected fish show similar behavioural signs to ich — flashing, rapid gill movement, lethargy, and loss of appetite — but often deteriorate more rapidly.
What causes it
A parasitic dinoflagellate with a lifecycle similar to ich. More dangerous than ich in its progression speed, as gill infestation causes rapid respiratory compromise. Commonly introduced on new fish, plants, or equipment from infected sources.
Treatment
Reduce or eliminate lighting — the free-swimming stage of Oodinium is photosynthetic and light-dependent. Reducing light slows the parasite's reproduction
Remove carbon from the filter
Treat with a copper-based medication or a proprietary velvet treatment — Waterlife Protozin and similar products are effective. Do not use copper in tanks with invertebrates
Raise temperature slightly within species tolerance to accelerate the lifecycle
Complete the full treatment course
Prognosis
Good with prompt, correct treatment. Poor if left untreated — velvet can kill fish faster than ich due to respiratory failure from gill infestation. Guppies and killifish are particularly susceptible and can decline very rapidly.
Fungal infections
Causative agents: Saprolegnia, Achyla and related water moulds
What it looks like
White, grey, or light brown cotton-wool-like tufts growing from wounds, fin edges, or areas of damaged tissue. Can appear on the body, fins, eyes, or mouth. Fungus also colonises fish eggs — infected eggs develop the same cotton-wool growth and will spread to healthy adjacent eggs if not removed promptly.
What causes it
Saprolegnia and related water moulds are present in virtually all aquariums but only colonise damaged or dead tissue. A fungal infection almost always means there was a pre-existing injury or problem — a wound from aggression, a lesion from bacterial infection, or damage from handling. Healthy tissue with an intact slime coat resists fungal colonisation effectively.
Treatment
Identify and address the predisposing injury or infection
Treat with an antifungal medication — Interpret Anti-Fungus, King British Fungus and Fin Rot Control, or methylene blue are standard options
If bacterial infection appears concurrent, use a combined antibacterial/antifungal product
For infected eggs: remove affected eggs promptly to prevent spread. Methylene blue added to hatching water at low concentration prevents fungal development on eggs
Prognosis
Good for mild to moderate cases treated promptly. Poor for severe infections covering large areas of body tissue or involving the gills.
Dropsy
Causative agents: Aeromonas hydrophila and related bacteria most commonly; may also be viral, parasitic, or organ failure
What it looks like
Severe abdominal swelling — the fish looks bloated, and when viewed from above, the scales are visibly raised outward, giving a pinecone-like appearance. The raised-scale presentation is the hallmark diagnostic feature and distinguishes dropsy from simple obesity or constipation, which don't cause scale raising. Additional signs include lethargy, loss of appetite, pale faeces, and protruding eyes.
What causes it
Dropsy is a symptom — specifically, kidney failure causing fluid accumulation in the body cavity. The underlying cause is most commonly a severe bacterial infection, but can also be viral infection, internal parasites, or organ failure from chronic poor conditions. By the time the raised-scale presentation is visible, significant internal pathology has already occurred.
Treatment
Dropsy is one of the most difficult aquarium fish conditions to treat successfully. Honesty is required here: by the time full raised-scale presentation is visible, the prognosis is often poor regardless of treatment. This doesn't mean treatment shouldn't be attempted, but realistic expectations matter.
Isolate in a hospital tank to reduce stress and prevent potential spread
Add Epsom salt (magnesium sulphate) at 1 tablespoon per 5 gallons — this acts as a mild diuretic and may help reduce fluid accumulation
Treat with a broad-spectrum antibacterial medication
Maintain excellent water quality in the hospital tank
If not improving within five to seven days or the fish appears to be suffering, euthanasia should be considered
Prognosis
Poor to guarded. Some fish recover from early dropsy with aggressive treatment; most fish with full raised-scale presentation do not. Catching the very earliest sign of bloating before scale raising occurs gives the best chance of recovery.
Skin and gill flukes
Causative agents: Gyrodactylus (skin flukes); Dactylogyrus (gill flukes)
What it looks like
Flukes are microscopic flatworm parasites invisible to the naked eye, making diagnosis by visual inspection alone difficult. Affected fish show: excess mucus production (a cloudy or opaque coating on the skin), flashing and scratching behaviour, rapid gill movement or laboured breathing, clamped fins, and lethargy. Gill flukes cause respiratory distress — the fish may gasp near the surface. Skin may appear irritated, red, or show patchy discolouration.
Flukes are commonly confused with ich (both cause flashing) or poor water quality (both cause surface breathing and lethargy). The key distinguishing features are: no visible white spots, and excess mucus rather than discrete spot lesions.
What causes it
Fluke introduction almost always comes from new fish, plants, or equipment from an infected source. They're particularly common in fish recently imported through the trade, wild-caught specimens, and fish from shops with high turnover and insufficient quarantine time.
Treatment
Praziquantel is the treatment of choice for both skin and gill flukes. Available as the active ingredient in proprietary treatments including Interpret Fluke and Tapeworm Tabs and similar products. Fenbendazole (sold as Panacur) is also effective.
Remove carbon from the filter
Treat the entire tank — not only the visibly affected fish
A second treatment two to three weeks after the first catches eggs that hatched after initial dosing — praziquantel is not effective against fluke eggs, only adult parasites. Do not skip the second dose
Prognosis
Good with appropriate treatment. Fish in respiratory distress from severe gill flukes need prompt attention — significant gill damage can cause lasting impairment even after parasites are eliminated.
Internal parasites and wasting disease
Causative agents: Hexamita, Camallanus worms, Capillaria and other internal parasites
What it looks like
Gradual weight loss despite normal or increased appetite — the classic "wasting" presentation. White or pale stringy faeces. Bloating in some cases. Lethargy and general loss of condition over weeks to months. In the case of Camallanus worms specifically: thin red thread-like worms visibly protruding from the fish's anus — an unmistakeable sign that requires immediate action.
What causes it
Internal parasites enter the aquarium on infected fish, live foods (particularly wild-caught tubifex and similar), and occasionally on plants or equipment. Wild-caught fish have significantly higher internal parasite loads than captive-bred specimens. Some species build up in the tank over time and infect additional fish as parasite numbers grow.
Treatment
Hexamita and Spironucleus (internal flagellates, associated with hole in the head and cichlid wasting): metronidazole, available in products including API General Cure or through a vet
Camallanus and other roundworms: fenbendazole (Panacur) is the most accessible treatment in the UK. Treat the whole tank as worm eggs contaminate the substrate
Capillaria: fenbendazole, often requiring a longer treatment course than for Camallanus
Prognosis
Variable. Fish caught early with good body condition generally respond well. Fish that have reached severe wasting — sunken belly, visible spine — have limited reserves for recovery. Act at the first sign of unexplained weight loss in a fish that appears to be eating normally.
Hole in the head (HITH)
Causative agents: Hexamita flagellates with contributing factors of water quality, diet, and activated carbon use
What it looks like
Small pits, erosions, or cavity-like lesions appearing on the head and along the lateral line. Initially presents as small white or mucus-filled pores, progressing to open pits over weeks to months. Most common in large cichlids — oscars, discus, flowerhorns — and less commonly in large catfish and other species kept in suboptimal conditions.
What causes it
The exact cause is debated. The most consistent contributing factors are: Hexamita flagellate parasites, chronic elevated nitrates, dietary deficiency particularly of vitamins C and D, and a controversial association with prolonged activated carbon use. Fish kept in excellent water conditions with varied, nutritious diets rarely develop HITH even when Hexamita is present — which suggests environmental factors matter as much as the parasite.
Treatment
Improve water quality substantially — increase water change frequency and volume, reduce nitrates below 20 ppm consistently
Remove activated carbon from the filter
Improve diet quality — add varied fresh foods, consider a vitamin supplement specifically targeting C and D deficiency
Treat with metronidazole to address the Hexamita component — API General Cure is the most accessible option in the UK
Prognosis
Good for early-stage HITH. Lesions typically stop progressing and heal over months once conditions improve. Advanced cases with deep pitting may leave permanent scarring but can stabilise. HITH is one of the conditions most responsive to husbandry improvement — getting the environment right matters more than medication alone.
Pop eye (exophthalmia)
Causative agents: Bacterial infection most commonly; may also follow physical injury or indicate systemic illness
What it looks like
One or both eyes protrude abnormally from the socket. The eye may appear swollen, cloudy, or in severe cases have a white ring around it. Unilateral pop eye (one eye) is usually a localised injury or infection. Bilateral pop eye (both eyes simultaneously) more often indicates systemic bacterial infection and carries a worse prognosis.
Treatment
Isolate in a hospital tank to reduce stress
Treat with a broad-spectrum antibacterial medication
Add Epsom salt at 1 tablespoon per 5 gallons to help reduce swelling through osmotic action
Maintain excellent water quality throughout treatment
Prognosis
Moderate. Unilateral pop eye from injury often resolves with treatment, though the affected eye may remain slightly enlarged or show some permanent opacity. Bilateral pop eye carries a guarded prognosis. Fish that lose an eye from severe pop eye can adapt to monocular vision and live well in low-competition environments.
Columnaris (cotton mouth / saddle back disease)
Causative agent: Flavobacterium columnare
What it looks like
White or grey lesions on the mouth, body surface, or fin bases — often with a cotton-wool appearance similar to fungal infection. A distinctive "saddleback" pattern of discolouration across the dorsal surface behind the dorsal fin is characteristic of some presentations. Mouth lesions appear as white eroded tissue around the lips. Columnaris can progress extremely rapidly — acute forms can kill fish within 24 to 48 hours.
What causes it
A gram-negative bacterium found in most freshwater environments. Disease occurs under stress — temperature stress in particular, as well as poor water quality and overcrowding. Livebearers, danios, and barbs are especially susceptible. Often introduced on new fish.
Treatment
Columnaris requires prompt action. Remove carbon, treat with a broad-spectrum antibacterial formulation. Reduce temperature slightly if within safe range for the species — columnaris is more virulent at higher temperatures. Salt baths can provide supplemental support for external lesions.
Prognosis
Highly variable. Chronic columnaris with slower progression responds reasonably to prompt antibacterial treatment. Acute columnaris with rapid deterioration and gill involvement can be very difficult to treat successfully. Speed of diagnosis and treatment matters enormously with this disease — if it's suspected, act immediately.
Medication notes — what not to do
A handful of consistent mistakes reduce treatment effectiveness or cause additional harm.
Never leave activated carbon in the filter during treatment. Carbon adsorbs medication from the water, neutralising it within hours. Remove carbon before dosing and don't replace it until treatment is complete and a water change has cleared medication residue.
Don't mix medications without specific guidance. Some combinations are safe; others produce harmful interactions or reduce effectiveness of both. If treating multiple concurrent conditions, research the specific combination or seek advice before mixing.
Don't underdose. Using less medication than directed to "be safe" often results in killing weaker pathogens while allowing resistant ones to survive — the aquarium equivalent of antibiotic resistance development.
Don't stop treatment early. Even if the fish looks recovered, complete the full course. Incomplete treatment allows partial pathogen recovery.
Never use copper-based treatments in tanks with invertebrates. Copper is acutely toxic to shrimp, snails, and crayfish at the concentrations required to treat fish disease.
Use reduced doses for sensitive species. Scaleless fish, loaches, and many catfish including plecos are sensitive to several common medications. Start at half dose and monitor closely before completing the course at reduced or standard dose.
Treat in a hospital tank where possible. Medicating a full display tank with established biological filtration, invertebrates, and sensitive species is riskier and less controlled than treating fish in a dedicated, simple hospital setup.
The role of quarantine
Every disease in this guide can be introduced to an established tank on a new fish. Every single one. A four-to-six-week quarantine period for all new arrivals — including fish bought online and shipped to you — catches the vast majority of disease presentations before they have the opportunity to infect fish you've kept for years.
A quarantine tank doesn't need to be elaborate. A bare-bottom tank of 40 to 60 litres with a cycled sponge filter, a heater, and some hiding places is entirely adequate. Run new fish through quarantine, observe daily for any signs of disease, and only transfer to the display tank after a clean period. If disease does appear in quarantine, you treat it in a small, controlled, fish-only environment without putting your entire collection at risk.
For fish bought online and shipped — including fish purchased through an aquatics marketplace — quarantine is particularly important. Shipping is inherently stressful, and stress suppresses immunity. A fish that appeared completely healthy when it left the seller may develop a disease presentation within days of arrival as immune suppression allows latent pathogens to take hold. The quarantine period protects both the new arrival, which benefits from a calm, low-stress recovery environment, and your existing fish.
The one consistent pattern among experienced fishkeepers who rarely lose fish to disease is not luck, expensive equipment, or any particular secret — it is simply that they quarantine every new fish, without exception. Not because they distrust sellers, but because they understand that disease introduction is a probabilities problem, and quarantine is what moves the odds decisively in your favour.
