Fish health
Sourced from veterinary and scientific references rather than from aquarium forums. Every condition here lists what else it could be and what observation separates them, because the most expensive mistake in fish health is confident treatment of the wrong thing.
Test the water before you do anything else
Ammonia, nitrite, chlorine, temperature and oxygen account for more aquarium deaths than every pathogen on this page combined. They act on every fish at once, they are invisible without a test kit, and the response to them is water rather than medication. Testing takes five minutes and changes the meaning of everything else you observe.
What you will not find here
No doses. AquariumHQ names classes of treatment and states what each can damage, but publishes no concentrations: product strengths differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of your tank survives are not on the label. AquariumHQ does not diagnose animals, and an aquatic veterinarian can do things this site cannot.
Environmental injuries — the ones that kill most fish (8)
- Ammonia poisoningAmmonia poisoning is chemical injury from ammonia in the water, most often in a tank that is not yet biologically established. The critical point is that a test kit reports total ammonia, but only the un-ionised fraction is seriously toxic — and what fraction that is depends on pH and temperature. The same reading is an emergency in alkaline warm water and a much smaller problem in acidic cool water.Act nowSuggestive by eye, not conclusive5 alternatives to excludeEnvironmental injury
- Nitrite poisoning (brown blood disease)Nitrite poisoning suffocates a fish in water that is full of oxygen. Nitrite is absorbed across the gill and converts haemoglobin into methaemoglobin, which cannot carry oxygen — so the fish gasps while the dissolved oxygen level is perfectly adequate. Chloride in the water competes with nitrite for the same uptake pathway, which is why it is protective.Act nowSuggestive by eye, not conclusive3 alternatives to excludeEnvironmental injury
- Chlorine and chloramine exposureChlorine and chloramine are added to drinking water specifically to kill microorganisms, and they damage fish gills directly on contact. The signs appear within minutes of a water change rather than over hours, which is the observation that identifies it. Chloramine is the more troublesome of the two because it does not disperse and because neutralising it releases ammonia.Act nowSuggestive by eye, not conclusive4 alternatives to excludeEnvironmental injury
- Oxygen deprivationOxygen deprivation occurs when dissolved oxygen falls below what the fish in the tank need. It is one of the few aquarium emergencies that can be substantially improved within minutes, by increasing surface agitation — which also makes it the most useful thing to try while you work out what is actually going on.Act nowSuggestive by eye, not conclusive5 alternatives to excludeEnvironmental injury
- Temperature shockTemperature shock is the physiological consequence of a rapid temperature change. Fish are ectotherms — their body temperature follows the water, and every enzyme system in them is tuned to a range. A rapid shift causes immediate distress and a period of immune suppression afterwards, which is why disease outbreaks so often follow a heater failure by a week or two.Act nowSuggestive by eye, not conclusive4 alternatives to excludeEnvironmental injury
- Gas bubble diseaseGas bubble disease occurs when water holds more dissolved gas than it can retain at ambient pressure, and that gas comes out of solution inside the fish — as visible bubbles in the fins, skin and eyes. It is uncommon, it is genuinely identifiable by eye, and it almost always points to a specific mechanical fault.Act todayRecognisable by eye4 alternatives to excludeEnvironmental injury
- Copper and medication toxicity in invertebratesCopper is lethal to shrimp, crayfish and snails at concentrations well below those used to treat fish parasites, and it is the single commonest cause of ornamental invertebrate death in aquaria. It enters tanks through medications, some plant fertilisers, snail treatments, and occasionally through the water supply itself.Act nowSuggestive by eye, not conclusive4 alternatives to excludeEnvironmental injury
- Shell erosion and pitting in snailsA snail's shell is calcium carbonate, and calcium carbonate dissolves in soft, acidic water. Erosion beginning at the spire and pitting across the whorls is a water-chemistry problem, not an infection, and no treatment will help while the water is still dissolving the shell.Act within daysRecognisable by eye4 alternatives to excludeEnvironmental injury
Appearances that are not single diseases (5)
- Fin rotFin rot is an appearance, not a disease: fins that are eroding, fraying or losing tissue from the edge inwards. It has several quite different causes, and the two commonest — poor water quality and damage from tankmates — are not infections at all. Treating it with an antibacterial without establishing which cause applies is the usual mistake.Act within daysAn appearance, not a confirmable diagnosis6 alternatives to excludeClinical presentation, not a single disease
- DropsyDropsy is not a disease. It is a description of a fish whose body has swollen with retained fluid until the scales stand out from the body like a pine cone. That appearance is the end stage of several unrelated processes — kidney failure, systemic infection, organ damage, a tumour — and the pineconing tells you that fluid regulation has failed, not why.Act todayAn appearance, not a confirmable diagnosis6 alternatives to excludeClinical presentation, not a single disease
- Swim bladder disorderSwim bladder disorder is a description of a fish that cannot control its buoyancy — floating, sinking or rolling. It is not one disease. The causes range from a full gut pressing on the organ, through selective breeding that has deformed it, to infection, injury and neurological damage, and they have completely different responses.Act within daysAn appearance, not a confirmable diagnosis6 alternatives to excludeClinical presentation, not a single disease
- Popeye (exophthalmia)Popeye describes an eye protruding from the socket. It is a sign, not a diagnosis, and the most useful first observation is whether one eye is affected or both: one eye usually means injury or a local infection, while both eyes more often point to a systemic problem or to the water.Act within daysAn appearance, not a confirmable diagnosis5 alternatives to excludeClinical presentation, not a single disease
- Constipation and intestinal blockageConstipation describes a fish whose gut is not emptying normally, usually presenting as abdominal swelling with flat scales, trailing or absent faeces, and reduced appetite. It is generally a feeding and temperature problem rather than a disease, and it is also over-diagnosed — several more serious conditions produce a swollen abdomen.Observe and monitorAn appearance, not a confirmable diagnosis5 alternatives to excludeClinical presentation, not a single disease
Syndromes with unsettled causes (5)
- Malawi bloatMalawi bloat is a rapidly fatal syndrome of African rift lake cichlids: abdominal swelling, loss of appetite and death within days. Its cause is not settled — diet, water quality, stress and flagellate infection are all implicated — but the association with feeding protein-rich food to species adapted to grazing algae is consistent enough to make diet the first thing to examine.Act todaySuggestive by eye, not conclusive4 alternatives to excludeSyndrome — signs cluster, cause is not established
- Hole-in-the-head and head-and-lateral-line erosionHole-in-the-head is a pattern of pitting and erosion on the head and along the lateral line, seen mostly in cichlids. It is a syndrome rather than a single disease: a flagellate parasite is frequently present, but nutrition, water quality and activated carbon have all been proposed as contributing factors, and the relative importance of each is genuinely unsettled.Act within daysSuggestive by eye, not conclusive4 alternatives to excludeSyndrome — signs cluster, cause is not established
- Bacterial septicaemia and ulcer diseaseSystemic bacterial infection in fish presents as red streaking in the fins and skin, open ulcers, reddened vents and rapid deterioration. Several bacterial genera produce an indistinguishable picture, and which one is present — and what it is susceptible to — cannot be established without laboratory culture.Act todayNeeds veterinary diagnosis5 alternatives to excludeSyndrome — signs cluster, cause is not established
- Chronic stressChronic stress is a sustained physiological state, not a mood. It suppresses immune function measurably, and it is the reason a great many aquarium disease outbreaks happen at all — the pathogens involved are usually already present and only become a problem when the fish's defences are down.Act within daysSuggestive by eye, not conclusive4 alternatives to excludeSyndrome — signs cluster, cause is not established
- Moulting failure in shrimpA shrimp that cannot complete a moult dies part-way out of its old exoskeleton. The associated sign keepers watch for is a white line across the back between carapace and abdomen. It is generally attributed to mineral availability and to abrupt parameter changes, and it is one of the few shrimp problems a keeper can genuinely reduce by managing water chemistry.Act within daysSuggestive by eye, not conclusive4 alternatives to excludeSyndrome — signs cluster, cause is not established
Diseases with an established cause (19)
- Whitespot disease (ich)Whitespot is a ciliate parasite that embeds itself under the fish's outer skin layer, where it appears as discrete white specks roughly the size of a grain of salt. The parasite is only vulnerable to treatment during the free-swimming stage of its life cycle, which is why treatment has to continue for days after the spots disappear — and why the spots vanishing is not evidence that it has gone.Act todayRecognisable by eye6 alternatives to excludeDisease with an established agent
- Velvet diseaseVelvet is a parasitic dinoflagellate that coats the fish in a fine gold or rust-coloured dusting, quite unlike the discrete spots of whitespot. It attacks the gills early and often kills before the skin signs are obvious, which makes it more dangerous than its reputation suggests and makes breathing difficulty a more important sign than colour.Act nowSuggestive by eye, not conclusive4 alternatives to excludeDisease with an established agent
- Gill flukesGill flukes are small monogenean flatworms that attach to the gill filaments with hooks. They cannot be seen with the naked eye, and the signs they produce — laboured breathing, flashing, one gill cover held open — are shared with ammonia poisoning, nitrite poisoning, low oxygen and several other parasites. A confident identification requires a gill scrape under a microscope.Act todayNeeds a microscope to confirm5 alternatives to excludeDisease with an established agent
- Skin flukesSkin flukes are monogenean flatworms that live on the body surface and fins rather than the gills. They are too small to see without magnification, and the flashing and greyish skin they cause is shared with several unrelated problems. Unlike the gill genera, the common skin genera bear live young rather than laying eggs, which changes how treatment has to be timed.Act within daysNeeds a microscope to confirm5 alternatives to excludeDisease with an established agent
- Anchor wormAnchor worm is a parasitic copepod, not a worm. The adult female embeds an anchor-shaped attachment into the fish's muscle and hangs out of the skin as a whitish thread, often with two egg sacs at the trailing end. The visible parasite is the last stage of the life cycle; the free-swimming stages in the tank are the ones that reinfect.Act within daysRecognisable by eye3 alternatives to excludeDisease with an established agent
- Fish liceFish lice are flattened, disc-shaped crustaceans a few millimetres across that attach to the skin with suckers and move about on the fish. They are visible to the naked eye, which makes them one of the few aquarium parasites that can honestly be identified by looking. They feed by piercing the skin, leaving wounds that commonly become infected.Act within daysRecognisable by eye3 alternatives to excludeDisease with an established agent
- Camallanus wormsCamallanus are intestinal nematodes whose females protrude from the fish's vent as short red threads. The red colour comes from the host's blood. By the time worms are visible the infection is established and usually present in other fish in the tank, including ones showing nothing.Act within daysRecognisable by eye4 alternatives to excludeDisease with an established agent
- EpistylisEpistylis is a colonial stalked ciliate that attaches to fish but does not feed on them — it feeds on bacteria in the surrounding water. It produces white tufts easily mistaken for whitespot, and it does not respond to whitespot treatment. Its presence usually indicates a high bacterial load in the water, and often an existing wound it has colonised.Act within daysNeeds a microscope to confirm3 alternatives to excludeDisease with an established agent
- Ichthyobodo infection (costia)Ichthyobodo is a tiny parasitic flagellate that infests skin and gills, producing a blue-grey film of excess mucus and heavy flashing. It is far too small to see, and the appearance it produces is shared with several other irritants. It is one of the more dangerous freshwater protozoa because heavy infestations can kill quickly, particularly in young fish.Act todayNeeds a microscope to confirm4 alternatives to excludeDisease with an established agent
- ChilodonellaChilodonella is a ciliate parasite of skin and gills that produces a greyish mucus film and can cause heavy losses quickly, particularly in cool water and in crowded fish. It cannot be identified without a microscope, and it is one of several organisms producing an identical appearance.Act todayNeeds a microscope to confirm4 alternatives to excludeDisease with an established agent
- ColumnarisColumnaris is a bacterial infection producing pale, greyish, flat lesions — classically a saddle-shaped band across the back, or a fraying, cotton-like growth around the mouth. It moves fast: in warm water it can kill within a day or two, and it moves faster the warmer the tank is, which reverses the usual advice about raising temperature.Act nowSuggestive by eye, not conclusive4 alternatives to excludeDisease with an established agent
- Water mould (Saprolegnia)The cotton-wool growth on a damaged fish is a water mould, not a true fungus — oomycetes belong to a different kingdom entirely. It is almost always a secondary coloniser of tissue that something else damaged first, which means the useful question is not how to treat the growth but what injured the fish.Act within daysSuggestive by eye, not conclusive4 alternatives to excludeDisease with an established agent
- Mycobacteriosis (fish tuberculosis)Mycobacteriosis is a chronic bacterial disease producing slow wasting, spinal curvature and sometimes ulcers, over months rather than days. It cannot be diagnosed without laboratory work, there is no practical cure in an aquarium setting, and — uniquely among the conditions here — it can infect the person keeping the fish through broken skin.Act within daysNeeds veterinary diagnosisCan affect people5 alternatives to excludeDisease with an established agent
- LymphocystisLymphocystis is a viral infection that produces irregular, cauliflower-textured growths on fins and body. It looks alarming and is usually harmless: it is generally self-limiting, resolving over weeks to months without treatment, and it is one of the few conditions where the correct action is to improve husbandry and wait.Observe and monitorRecognisable by eye4 alternatives to excludeDisease with an established agent
- Dwarf gourami iridovirusDwarf gourami iridovirus is a systemic viral disease with high mortality, associated particularly with commercially produced dwarf gouramis. There is no treatment. Its practical significance for a keeper is a purchasing decision rather than a treatment one: a substantial proportion of farmed dwarf gouramis have been reported to carry it.Act within daysNeeds veterinary diagnosis4 alternatives to excludeDisease with an established agent
- Neon tetra diseaseNeon tetra disease is caused by a microsporidian parasite that forms cysts in muscle, producing patchy loss of colour that looks like pale areas beneath the skin. It cannot be distinguished by eye from a bacterial condition producing an almost identical appearance — which is why the veterinary literature has a name for that second thing: false neon tetra disease.Act within daysNeeds veterinary diagnosis4 alternatives to excludeDisease with an established agent
- ScutariellaScutariella are small white flatworms that attach around a shrimp's head and rostrum, where they look like tiny white threads or hairs. They are visible on a close look, are specific to shrimp, and unlike most things affecting shrimp this one is genuinely a parasite rather than a chemical injury.Act within daysRecognisable by eye3 alternatives to excludeDisease with an established agent
- Vorticella and stalked ciliates on invertebratesVorticella are stalked ciliates that attach to a shrimp's exoskeleton and look like a white or greyish fuzz, frequently mistaken for fungus. Like Epistylis on fish, they feed on bacteria in the water rather than on the animal, so their presence is a signal about the water rather than about the shrimp.Observe and monitorNeeds a microscope to confirm3 alternatives to excludeDisease with an established agent
- Chaetogaster on snailsSmall pale worms moving on a snail's shell and body are usually Chaetogaster, an oligochaete that lives on snails and feeds on small organisms in the water around them. It is generally regarded as commensal rather than harmful, and its presence is more informative about the tank than about the snail.Observe and monitorNeeds a microscope to confirm4 alternatives to excludeDisease with an established agent