The A-Z of Fish Health, Part 4: S-Z


Dr Peter Burgess, Senior Consultant of the Aquarian Advisory Service, takes us through the alphabet, looking at the common – and not-so-common – health matters that can affect our fish.





Genus of aquatic water moulds (aquatic fungi). This is a common cause of fungus infections of freshwater fish and fish eggs. Manifests as one or more white-grey cotton wool-like growths that project from the body surface, such as skin, fins, eyes or gills. Typically, only a single fish is affected.  

Treat with a proprietary fungus remedy and some products contain malachite green and formalin as active ingredients. Salt-tolerant fish can be treated with aquarium salt (1-3 grams per litre). Novel fungus remedies include bronopol, marketed as Pyceze, and Pimenta-extracts (for example. Pimafix).  

Saprolegnia rarely attacks healthy, unstressed fish, so investigate why fungus initially took hold. Possibilities include unhygienic water conditions, overcrowding, skin injuries or infections, poor diet or exposure to water temperatures below fish’s tolerance range.   

Secondary infection

An infectious organism having invaded tissues or organs already damaged by another (primary) infection. For example, a whitespot parasite infection will damage and breach the fish’s skin, making the skin vulnerable to secondary bacterial infections. In many cases, these secondary infections ‘finish off’ the fish if not treated in time.  


As fish age they begin to lose condition and may develop organ malfunctions and skeletal problems such as curvature of the spine.

Their immune system becomes less efficient, making them prone to infections such as finrot and tumours. The ageing fish may become thin and its colours fade.  

Ensure that senile individuals are still able to compete for food and are not the victim of attack by tank mates and, if necessary, remove frail specimens to a peaceful ‘retirement’ aquarium.


Blood poisoning caused by the presence of toxin(s) in the blood. Most cases involve toxins produced by pathogenic bacteria (such as various species and strains of Aeromonas, Pseudomonas and Vibrio) that have entered the fish’s bloodstream. Septicaemia can manifest as widespread reddening of the fish’s skin and sometimes enlargement of the blood vessels (hyperaemia) that run through the fins. In some cases it shows up as brown blotches on the skin (as sometimes observed in affected goldfish).  

Septicaemic fish may appear ill,  lethargic and are likely to die unless promptly treated with antibiotics from the vet. Over-the-counter bacteria remedies are less effective. Good aquarium hygiene will reduce risks of bacterial infections that can lead to septicaemia.

Skin parasites

Important skin parasites of fish belong to four groups:

  1. Protozoa (such as Ichthyophthirius – whitespot; Piscinoodinium – velvet; Trichodina)
  2. Flukes (notably Gyrodactylus species)
  3. Crustaceans (Lernaea – anchor worm; Argulus – fish louse)
  4. Leeches (Hemiclepsis and Piscicola) generally only a problem in ponds).

Most skin parasites attach to fish to feed on mucus or skin cells.   A few, such as the fish leech and fish louse, use piercing mouthparts to puncture a skin capillary for a blood meal.

Harm caused by skin parasites

Individually, most skin parasites are  too small to cause significant harm.  But in large numbers their collective damage can become life threatening.  Some irritate or physically damage the fish’s skin.  The tiny Trichodina has a circular array of sharp ‘denticles’ that abrade the skin surface as it moves over the fish.  Equally menacing are the curved hooks that skin flukes use to attach to their host.  

The whitespot parasite employs tissue-digesting enzymes and physical force to reach its site within the skin. The blood-sucking fish louse causes deep feeding wounds as well as abrasions with its spiny leg-like appendages.  

These forms of parasite damage cause breaches to the skin’s protective barrier, rendering it highly vulnerable to infection by bacteria, fungi, and other pathogens.  It is often these ‘secondary’ infections that eventually kill the fish.


Physical damage and tissue-digesting enzymes irritate the fish’ skin which may react by secreting copious mucus, causing the affected areas to take on a greyish or milky appearance (‘slimy skin disease’).  

Affected fish may exhibit signs of irritation, such as repeated body rubbing, jumping, darting, or actually leaping from the water.  Any outward symptoms, such as white spots, may help to identify any skin parasite involved.

Treating skin parasite problems

Fortunately, numerous over-the-counter remedies are available.  Most types of skin parasite have life cycles that include free-living stages in the water or substrate — so as a general rule medicate the whole aquarium or pond in order to completely eradicate them.

Scale erection (raised scales)

In this abnormal condition some or all of the fish’s scales project outwards, giving a serrated or 'pine-cone' appearance to the affected area of skin.

Scale erection may be easier to detect when the fish is viewed directly from above.  Where accompanied by bodily bloating (and possibly bulging eyes), this signifies a potentially lethal condition known as dropsy which is sometimes caused by a bacterial infection that may require antibiotic treatment

Where just a few scales are raised, this may be due to an injury or perhaps a localised bacterial or parasitic infection.


Curvature of the backbone which becomes kinked  (snake-like curvature). Typically, scoliosis develops slowly. Possible causes include a genetic or developmental defect, infection, nutrient deficiency, or simply old age. There’s no cure.


This term describes an abnormal ‘swimming on-the-spot’ behaviour in which the fish flexes its body from side to side but without moving forward in the water. It may be interspersed with normal swimming.  

Any display of shimmying may be as the result of damage being caused to the fish’s nervous system and affecting coordination. It often accompanies a bacterial infection known as columnaris and caused by Flavobacteria.    

Treatment relies on diagnosing and dealing with the underlying cause, for example bacterial infection.

Slimy skin disease

Condition in which skin produces excess mucus. Affected area(s) may appear milky or greyish. Copious mucus is produced in response to an irritation as caused by skin parasites and pathogens, or adverse water conditions such as extremes of pH or chemical irritants.


Genus of protozoan (single-celled) parasites. Spironucleus are common gut inhabitants of various fish, including cichlids, but generally cause no harm. These occasionally invade tissues which can lead to disease. Spironucleus vortens has been implicated in cases of Hole-in-the-head syndrome in cichlids. Spironucleus species have also been associated with low hatching rates and poor fry survival in Angelfish breeding units. Treat with metronidazole or similar from vets.


This can have many causes, such as  genetic or developmental abnormality, improper diet, or chronic stress. Chronic disease can also retard growth. In any large brood  it is normal for some to remain stunted. Where a dietary problem is suspected, a complete, well-balanced diet can sometimes improve stunted individuals.


Spring Viraemia of Carp (SVC) is a highly contagious and potentially life-threatening viral disease. It primarily affects Koi and Common carp, but can also occur in other coldwater cyprinids and Wels catfish (Silurus glanis).  

Although rare in the UK, SVC can be brought in on infected fish, especially those illegally imported from SVC-affected regions. The disease typically manifests in spring, above 7°C/45°F water temperature. Symptoms may include skin ulceration, bloating, loss of balance, darkening, haemorrhaging of the skin, pop-eye, and trailing faeces.  

If SVC is suspected, contact a vet or Koi health professional immediately. It is a notifiable disease and there’s no chemical cure.

Affected stock may have to be put down.

Swimbladder disorders

The swimbladder is a gas-filled sac within the body cavity and functions primarily as a buoyancy aid. Infection, displacement or damage can lead to its enlargement, resulting in excessive positive buoyancy — fish floating at surface. It may become reduced in size, collapsed, or fluid-filled, resulting in negative buoyancy  — fish sinking to bottom of tank.  

Goldfish seem particularly prone to over inflation or displacement of the swimbladder, which may be due to disease, abnormal kidney enlargement and other causes.

Swimbladder disorders and the buoyancy problems they cause are generally difficult to treat.


There are some 3,400 species of these worm-like parasites known technically as cestodes and all are parasites of various animals. Of these, some 800 species parasitise various freshwater and marine fish.  

Some species occur as the adult stage within the fish’s intestinal tract where they absorb ingested food from their host. An example is Khawia that lives within the intestines of carp.  

Adult tapeworms are typically elongate and ribbon-like, comprising numerous segments.  Some species may reach 30cm/12”. The adult has hooks and suckers for attachment to the gut wall, so preventing it from being flushed out of the host.

Other tapeworm species occur as the larval stage within the fish, typically residing within the body cavity.  An example is Ligula that occurs in carp and other cyprinids and the adult worm lives in the intestines of fish-eating birds, such as gulls. Paradoxically, some tapeworm larvae are bulkier than the adult worm!

Most tapeworms have highly complex life cycles in which they must sequentially pass through two or more different hosts to complete each generation. Depending on tapeworm species, the adult may live in the gut of either a mammal, bird, or fish.  

Fish may acquire tapeworms by eating an aquatic worm — Tubifex for example — or aquatic crustacean, such as a copepod, that happens to be infected with tapeworm larvae.

Because their life cycles require two or more different hosts, tapeworms are not common in farmed ornamental fish and are highly unlikely to be infectious under aquarium conditions. Wild caught fish, on the other hand, may harbour these parasites. Collectors and exporters use chemicals to purge wild fish.

Fish lightly infected — perhaps harbouring just a single worm — may exhibit no obvious symptoms. A heavy infestation of gut tapeworms can consume a lot of the fish’s food intake, resulting in it slowly becoming weakened and starved from within. The sheer bulk of several large tapeworms can cause a noticeable abdominal swelling.

In live fish, it is difficult to confirm a tapeworm infestation except by having a sample of the fish’s faeces examined professionally for tapeworm eggs. Larval tapeworms do not produce eggs.  

Gut-dwelling tapeworms can be purged using medications, known as anthelmintics — an example being Praziquantel. These are available from a vet, but some may now be legally obtained without prescription from special aquatics suppliers. Traditional parasite cures —for example, whitespot cures — will not kill tapeworms.  

Temporary parasites

These spend only short periods of time on the fish. An example is the fish leech (Hemiclepsis) that attaches to the skin in order to feed on blood. For much of the time, the leech lives freely in the water.  

Another temporary parasite is the fish louse (Argulus) which is a type of crustacean.


The genus of protozoan (single-celled) organisms, two Tetrahymena species, T. corlissi and T. pyriformis, have been linked with disease in livebearers and other aquarium fish.  They cause ‘Guppy disease’ (‘Tet disease’).

These microscopic organisms are generally free-living but, under certain circumstances, may parasitise the fish’s skin and sometimes deeper tissues.  

Tetrahymena are a major health problem on some guppy farms.  Congregations on the fish may result in whitish skin patches and most evident on dark fish such as Black mollies.

Affected scales may protrude outwards, resulting in localised area(s) of bristly-looking skin.  

In guppies, parasites may form a ring around the eye, causing ‘spectacle eye’ disease and heavy infections can kill guppies within 24 hours. Treat outbreaks with a general skin parasite cure or whitespot cure.  

Unhygienic water conditions, overcrowding, and exposure to low water temperatures have all been linked with Tetrahymena outbreaks.


These microscopic saucer-shaped protozoa occasionally attach to the  fish’s skin or gills. They are more commonly encountered on wild or pond fish and may be present in large numbers on individuals already diseased or weakened. Severe outbreaks cause skin and gill damage, risking secondary bacterial and fungal infections.

Badly affected fish may show signs of skin irritation such as repeated body rubbing and, in severe cases, the fish may hang listlessly below the water surface. Treat using a commercial skin parasite remedy.


This is the common name for a neoplasia. A tumour is an unnatural growth that sometimes develops on skin or within the fish’s tissues or organs. Some types are more harmful than others.

They are generally unpreventable and incurable, but most are non-contagious.


Localised crater-like lesion on the surface of an organ or tissue. Skin ulcers appear as one or more open sores, typically red-brown with a whitish border.  A deep skin ulcer may extend to the underlying muscles.  

Ulcers can be caused by raised ammonia levels. In other cases, a bacterial infection is to blame. Some have a viral origin. Infected skin ulcers usually respond to anti-bacteria remedies, including herbal (Melaleuca-based) products sold for use on fish.

Those not healing properly may require antibiotics from the vet. Recuperate ulcerated fish under clean, hygienic water conditions to reduce the risk of further infections of the damaged area.

Velvet disease

Oodinium/rust disease/gold dust disease is caused by tiny protozoa belonging to the genera Piscinoodinium  (freshwater form of velvet) and Amyloodinium (marine velvet).  

The parasitic stage anchors itself to the skin, fin or gill, causing irritation. The damaged skin is vulnerable to secondary bacterial or fungal infections. Gill damage can result in severe breathing problems and flared gill covers.

Fish with heavy infections — involving many hundreds or thousands of parasites — appear covered in a fine dusting of tiny spots; each being a single parasite.  In freshwater velvet the spots may appear green-gold under bright light.  

Affected fish tend to body rub and twitch fins and each parasite eventually exits the fish and transforms into a reproductive stage in the aquarium.  

Within a matter of days numbers can increase dramatically.  

Velvet disease is highly contagious and prompt treatment is necessary to eradicate parasites before they reach deadly levels. Commercial velvet and general parasite cures are on the market to deal with outbreaks.

With Piscinoodinium it also helps to darken the tank during an outbreak, as these parasites rely partly on photosynthesis for energy.

Vertical transmission

This involves the transmission of a pathogen, notably certain bacteria and viruses, from parent fish to offspring via the gametes or reproductive system. For example, a female fish with mycobacteria may pass on some of bacteria to her developing fry or eggs, such that the offspring are born (or hatch) already infected.     

Viral diseases

Viruses are primitive disease-causing organisms that live within cells of their host.  Most are considerably smaller than bacteria, so cannot be seen under an ordinary light microscope — a costly electron microscope is required instead.  

Numerous types of virus affect fish, causing a wide range of diseases that vary in symptoms, severity and contagiousness.  These include KHV (Koi herpes virus), Carp pox virus and Lymphocystis virus.

Some viruses are highly host specific, meaning they infect only one or a few species of fish. For example, there is an iridovirus that appears to affect only Ram cichlids.  Others have a broader host range, such as the fairly common Lymphocystis virus that causes harmless skin lumps on cichlids, gouramis and other ‘evolutionary advanced’ groups.  

There are currently no chemical treatments to cure infections in fish – even antibiotics won’t work. In some cases, the fish’s own immune system may overpower the virus, leading to self-cure, as generally happens in Lymphocystis.

Wasting disease

Chronic, debilitating disease in which the fish becomes progressively thinner — wasting away.  Usually attributed to a mycobacterial infection.  


This common and easily recognised disease is caused by single-celled protozoa that attacks skin, fins and gills.

The parasite that affects freshwater fish (tropical and coldwater species) is Ichthyophthirius. Cryptocaryon affects tropical and sub-tropical marine fish.

Parasites burrow beneath the surface of the skin causing white-grey spots — each one growing to roughly the size of a sugar grain. The mature parasite later exits the fish and forms a reproductive  ‘cyst’ within the substrate. Each one gives rise to many hundreds of microscopic infective stages that swim and search for fish to infect, so repeating the life cycle.

Whitespot is potentially lethal, causing extensive skin and gill damage. Affected fish may flick fins and rub their bodies. Heavy infections may cause breathing problems — gasping and fast gill beats — and skin damage may lead to secondary infections. Badly affected fins become ragged.  

These two parasites’ reproductive potential, combined with their broad host range, means that whitespot can quickly overwhelm all fish in an aquarium or pond.

Promptly treat aquarium or pond with a commercial remedy. Treat all fish, even if only a few show signs of disease. One or more repeat doses may be necessary, especially when dealing with Cryptocaryon, to ensure complete eradication.

Deal with any secondary bacterial or fungal infections with an appropriate remedy. Do not use copper-based cures for aquariums that house invertebrates. Seek advice in those instances.


Can be transmitted from animals, including fish, to humans — or vice-versa. A few types of bacteria, including species of mycobacterium, salmonella and aeromonas, have very occasionally passed from fish, or their water, to humans. Fortunately, the risk of contracting a zoonotic disease from pet fish is extremely low.  

Attention to personal hygiene will greatly reduce any risk. Thoroughly wash your hands after handling fish and avoid placing unwashed hands in the mouth or eyes. Bear in mind that fish are among the safest of pets, which is why we often see aquariums in hospitals and retirement homes.




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