The A-Z of Fish Health, Part 2: F-L

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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.

A-E

M-R

S-Z

Fatty liver disease

Nutritional disease. Often linked to feeding diets containing saturated fats such as fatty red meats. Within a warm-blooded animal, these saturated fats are in their liquid state, making it easy for the animal to process and store them in the liver. But if fed to fish, these fats remain solid within the fish’s relatively cold body (this applies to tropical fish also) making it difficult for the fish to properly digest and store them. As a result, the fish’s liver may be damaged and its many important functions, such as glycogen storage, impaired.  

Other causes of fatty liver degeneration include dietary deficiencies, for instance from foods lacking in methionine or certain fatty acids. Feeding diets containing rancid (oxidised) fats can also cause liver damage.

Fatty liver problems generally go undetected as the condition causes no obvious outward signs. Liver damage will, however, affect the fish’s general health and growth and is potentially life-threatening. This condition is preventable by feeding only quality fish foods that are stored correctly. Avoid giving fish animal meats or other foods containing animal or dairy fats, eg burgers, corned beef, sausages, hams, cheeses, biscuits and the like.

Fenbendazole

Drug used to treat certain roundworm infestations of the gut. Fenbendazole is one of the drugs of choice for treating Camallanus gut worms that sometimes affect livebearers, eg Mollies, Platies and other fish. Can be delivered via the water as a long-term bath, eg 2 mg/l added on days 1, 8 and 15, or incorporated with the fish’s food.

Fibroma

A harmless benign tumour, composed mostly of fibrous tissue. Very uncommon. Skin fibromas appear as raised lumps, but can easily be confused with other more common types of skin tumours, eg papillomas, and other causes. No treatment and generally these are best left alone.  

Fin rot

Erosion of the fin. This condition can affect any fin, however the tail fin seems more prone to this disease in which case it is specifically referred to as ‘tail rot’. Initial signs are whitish areas appearing on the fin, or there may be whitening and erosion of the fin edge, giving a ragged appearance. As the disease progresses, the fin membrane rots away, leaving the fin rays exposed. With tail rot, the infection may spread to the adjacent body, causing the affected tissue to become very pale, sometimes with a pinkish blush. Advanced tail rot causes the fish swimming difficulties and is life-threatening.

Various types of bacteria have been linked with fin rot, in particular Flavobacterium columnare, formerly known as Flexibacter. These bacteria tend only to attack fish that have a weakened immune system or those that have already sustained fin damage, perhaps due to injury or parasite attack. Unhygienic water conditions also render fish more vulnerable to fin rot. Transportation stress is another contributory factor, which is why outbreaks of fin rot are not uncommon in recently imported stock.  

Fin rot is generally curable if caught early, before it spreads to the body. Treat with a general bacteria remedy or fin rot cure. With advanced infections that involve the body, even the most effective treatments, such as antibiotics, cannot be guaranteed to save the fish. Fin rot is not highly contagious and often only a single fish is affected. It is, however, wise to isolate affected fish and treat them under very hygienic conditions. Preventative measures rely on providing fish with optimal water conditions and minimal stress.

Note: Some fish possess fins that normally appear tattered or frayed. This is particularly the case with adult males of certain species, such as the swordtail characin, Corynopoma riisei, and Arulius barb, Puntius arulius.

Certain cultivated varieties of fish have been selectively bred for their ragged fins, a recent example being the Crowntail fighting fish, Betta splendens.  

Fin twitching

It is normal for fish to occasionally twitch or flick their fins, perhaps in response to a transitory irritation or as means of communication. However, when fin twitching occurs frequently, this can indicate infestation with skin parasites, such as Trichodina, Ichthyophthirius or skin flukes, to name a few, particularly if accompanied by frequent rubbing of the flanks against submerged objects, a behaviour known as 'flashing'.

Poor water quality can also make fish flick or twitch their fins, hence water tests should be carried out as part of the investigation.  

Flukes

Group of parasitic flatworms. They are fairly large multi-celled creatures, the majority ranging between 0.5 and 3 mm long. Many flukes are parasites of fish. Some target the fish’s skin, fins or gills, whereas others live within its body or blood. There are two main groups of flukes: the monogeneans and the digeneans.

Monogenean flukes pose the greatest health risks to aquarium and pond fish, examples being the skin flukes, eg Gyrodactylus species, and gill flukes, eg Dactylogyrus species. They can be highly contagious. Various disease remedies are available for dealing with outbreaks of skin or gill flukes.

Digenean flukes that occasionally affect fish are the eye-flukes, eg Diplostomum, and blood flukes, notably Sanguinicola. Digeneans have complex lifecycles that may include a bird and an aquatic invertebrate, eg aquatic snail, as well as a fish. Hence, digenean infections are more commonly encountered in wild or pond-raised fish.

Most digenean fluke infections are untreatable, but fortunately are rarely life-threatening and are generally non-contagious under aquarium conditions.        

Fish louse (Argulus)

Blood-sucking skin parasite characterized by its disc-like shape and visible size, reaching 1 cm/1/2” in diameter. Argulus is not a true louse but a primitive member of the crab family Crustacea. There are several species of Argulus, the most common being Argulus foliaceus. They can affect a wide range of fish species but are mostly encountered in ponds rather than aquariums. Argulus are known as temporary parasites because they spend only some of their time attached to the fish’s skin in order to take a blood meal. At other times they live freely in the water.  

Despite its large size, Argulus can appear well camouflaged against its host’s skin, due partly to its semi-transparent body: it can be mistaken for a partly detached scale. Heavy infestations, which are uncommon, may cause white patches on the fish’s skin. The parasite’s feeding wounds develop into small red spots that can become infected.

Badly infested fish exhibit listlessness, anaemia, and loss of appetite. Where Argulus is established in the pond, outbreaks tend to occur in spring and summer. Individual Argulus can be carefully removed from the fish using tweezers. Special anti-crustacean treatments, from a vet, are required to fully eradicate the adult and larval stages from the pond.

Flagellates

Group of single-celled protozoa.  Many are harmless free-living organisms, but some parasitic species affect fish. They possess one or more whip-like structures, known as flagellae, that aid swimming. Important examples are the skin-parasitic flagellates that cause velvet disease in freshwater and marine fishes (caused by Piscinoodinium and Amyloodinium parasites, respectively). Others, such as the trypanosomes, parasitise the fish’s blood instead.

The gut-dwelling Hexamita and Spironucleus flagellates occasionally cause disease in various fish, including hole-in-the-head disease in cichlids. Over-the-counter treatments are available for dealing with velvet disease. Internal flagellates are more difficult to treat, however the drug metronidazole, available from the vet, is used to cure hole-in-the-head.    

Flashing

Behaviour in which the fish rubs its body, usually its flanks, against a submerged object such as a rock, stone, or plant stem. In the wild, when the fish rolls on its side to rub itself, this may cause a flash of sunlight to reflect off its silvery flanks – hence the term “flashing”. Repeated flashing often indicates a skin irritation, such as caused by skin parasites, eg skin flukes, whitespot parasites, Trichodina, etc. 

Flavobacterium infections

Formerly known as Flexibacter, these can cause serious skin and internal infections in aquarium and pond fish, and outbreaks are more likely at water temperatures above 15ºC/60ºF.

They are a major cause of fin rot whereby one or more fins, typically the tail, erode away — and also cause mouth rot.

Skin infections may lead to localised bruising and, where the gills are involved, can cause respiratory problems.

Flavobacterium look distinctive under a powerful microscope as long, thin, rod-shaped organisms that tend to congregate in stacks. Individual bacteria glide against each other within the stack, so are sometimes called 'gliding bacteria'.

Over-the-counter anti-bacterial treatments are available for mild cases, including fin rot and mouth fungus, but stubborn or serious infections may require antibiotics from the vet.

Formalin

A solution (37-40%) of formaldehyde gas, this is traditionally used to treat skin and gill parasites, (eg whitespot and Trichodina protozoa and flukes).  It also has slight anti-bacterial activity and is sometimes used in combination with malachite green, notably for treating whitespot, Ichthyophthirius.

Formalin reduces oxygen levels in water, so ensure good aeration during treatment. It is potentially toxic to fish and aquatic plants, so use with care and never overdose.

Fish species vary in sensitivity to Formalin and contact with human skin must be prevented. Wash it off thoroughly and seek medical advice if eyes are affected.

Fungus diseases

Various aquatic fungi or water moulds are capable of infecting freshwater fish. The best-known example is Saprolegnia fungus which forms white-grey cotton wool-like growths on the body surfaces of fish as well as on eggs. Achlya and Aphanomyces are other water moulds causing similar symptoms.

Water moulds rarely attack uninjured, healthy fish, so address any underlying cause, (eg physical injury, parasite damage or poor water conditions) in addition to treatment.

Various fungus treatments are available, including salt (for salt-tolerant fish) and botanical formulations (eg Pimenta extracts). Being salt-intolerant, water moulds are not a problem in marine systems.

Other fungus-like pathogens of fish include Branchiomyces, which occasionally affects pond fish, notably carp, causing gill damage and breathing difficulties. Dermocystidium is another fungus-like organism which manifests as nodular swellings on skin and fins. There are no known chemical cures for either of these organisms.

None of the above organisms are true fungi, but they possess fungus-like characteristics — namely branching, thread-like filaments (hyphae) and tiny spores. To add to the confusion, mouth fungus, which causes the fish’s mouth to erode, is actually a bacterial disease and generally caused by the Flavobacterium species.

Gas bubble disease

Potentially fatal, similar to 'the bends' among divers, and characterised in fish by small gas bubbles (emboli) varying in size within the skin, eyes and blood. Affected fish may become lethargic, but often show no other symptoms. Cause is supersaturation of gases, notably nitrogen, within the water and can occur when cool, gas-rich water is suddenly heated, as when a new aquarium is filled with cold water which is quickly warmed to operating temperature.  

It can also arise from leaks within high-pressure pipework or connectors (eg those serving pumps and filters), causing air to be sucked into the system. If suspected, immediately increase aeration to help drive off excess gas.

There is no specific treatment for affected fish, but less serious cases may recover if removed from the conditions.   

Gasping

Illustrated by abnormal behaviour involving laboured and/or rapid opening and closing of the mouth, in many cases at the water surface. This reflects a breathing problem and is usually indicative of oxygen starvation.

Among several possible causes a lack of oxygen is often blamed, perhaps due to fish overstocking, pollution, and/or inadequate aeration.  

Another cause is a gill infection, commonly due to parasite or bacteria attack. Chemical damage to gills — for example, due to exposure to high levels of ammonia wastes or chlorinated water — can also cause gasping. 

Suspect a water problem (eg low oxygen levels or chemical poisoning) when several previously healthy fish suddenly gasp, before considering gill infection.  Gasping should be investigated immediately, otherwise the fish may die, and ideally aeration increased as soon as possible.  

A quick fix for pond fish involves running a hose to the pond and spraying the surface to force air and oxygen into the water. Increased aeration will help while the underlying cause is investigated and remedied.

Gill maggots

The common name for gill-parasitic crustaceans belonging to the genus Ergasilus.

Gill problems

Gills perform several functions, including respiration, excretion and osmoregulation (the regulation of internal salts and water). Any damage or disease is therefore potentially life threatening.  

As gills are in constant contact with water they are directly exposed to any harmful chemicals within it. Their rich blood supply and ease of access also makes them a target for various parasites and pathogens.

Problems that can affect aquarium and pond fish include:

Damage by water-borne poisons or toxins: Gills can be harmed by high levels of ammonia wastes and chlorine and chloramine disinfectants in untreated tapwater. Their protective surfaces may become stripped if exposed to soaps and detergents, so never use these products to clean aquariums, décor or submersible equipment.  

Avoid immersing bare metal objects in the aquarium or pond as exposure to high levels of heavy metals (notably zinc, copper, mercury, cadmium and lead) can also harm gills.

Many insecticides and herbicides can damage gills and other organs. Applying garden chemicals near the pond, especially on windy days, could allow tiny droplets into the water and poison the fish.

pH damage: Fish vary between species in pH tolerance; a particular value may be healthy and optimal for one type but damaging to the gills and other organs of another. In many species gills may become irritated and damaged if the pH becomes very acid, below about pH 5.5, or very alkaline, above pH 9.0.

Gill infections: A wide range of organisms infect gills. They include certain viruses (eg Koi herpes), bacteria (such as Flavobacterium which causes bacterial gill disease in pond fish), fungi (eg Saprolegnia), parasitic protozoa (eg Ichthyophthirius — the cause of whitespot), Piscinoodinium — velvet disease (Trichodina), flukes (Dactylogyrus — gill flukes) and crustacean parasites (eg Ergasilus — gill maggots).

Affected fish may exhibit laboured and/or fast opening of the mouths and gill covers. They may remain at the surface, gasping and gulping.

Remedial action: First perform water tests (eg pH, ammonia levels) before considering gill infection — and refer to the section on gasping for problems relating to low oxygen levels.  

Look for signs that could point to gill parasites or gill pathogens. For example, many gill parasitic protozoa also attack the skin, causing repeated body rubbing and sometimes other symptoms (eg white spots).  

Treat gill infections with an appropriate remedy or, if in doubt, seek professional help.

Gold spot disease

Alternative name for freshwater velvet disease. Caused by skin parasites of the genus Piscinoodinium, these contain chlorophyll pigment for photosynthesis, which gives them a golden colour when viewed in direct light — hence 'gold spot' disease.

Velvet disease will be covered in more detail under its own A-Z entry.

Growths

These manifest themselves as lumps, cysts, spots, or other swellings. They are usually noticed only when occurring on the body surface, although internal growths can also occur.

Some skin growths are the same colour as surrounding skin, while others have contrasting pigmentation (cream, white, pink and red, for example). Their shape, texture and ultimate size will vary according to the underlying cause.

Many abnormal skin and fin growths are harmless, but some are potentially life-threatening, so properly identify them to gauge their severity. If in doubt, have the affected fish examined by a vet or fish health professional.

Causes of abnormal growths include:

Fungus infections: Growths appear as white-grey cotton wool-like tufts that stick outwards from the body surface. Treat with a fungus remedy added to the water.

Parasite cysts: Certain parasites cause raised growths on the skin and/or fins. For example, whitespot parasites (Ichthyophthirius in freshwater, Cryptocaryon in marine) appear as sugar-grain-sized white cysts just under the skin.

In Koi, a relatively harmless but untreatable disease caused by Dermocystidium parasites manifests as raised lumps on the skin or fins, and sometimes the gills. Each lump can become pea-sized (up to 1cm/0.4” across) and eventually burst, releasing tiny spores into the water. Over-the-counter treatments are available for eradicating many common skin parasites.

Tumours: More correctly known as neoplasias, these occur when one or more cells within the body malfunction and begin to grow and multiply out of control. The abnormal cell mass may ultimately grow to a visible lump of a size, shape, colour and texture depending on the type of cells involved. Typically, tumours grow slowly and in many cases the fish appears otherwise normal and healthy.

Most tumours are best left alone and there are no chemical treatments. In exceptional cases, a vet may decide to remove a tumour on the skin or fins, particularly if it interferes with the fish’s vital functions; for example, where it encroaches on the vent or mouth.

Bacterial granulomas: A granuloma is a non-cancerous growth caused by chronic inflammation.

Several types of bacteria, notably mycobacteria and mocardia, may cause granulomas of the skin. These typically also invade internal organs and tissues, making affected fish appear weak and in ill health, and they may die if untreated.

Treatment is problematic. Antibiotics from the vet generally offer the best hope, but even these prescription-only drugs may fail.

Wherever possible, permanently isolate the affected fish. Assess overall aquarium/pond hygiene as dirty water conditions will render fish more susceptible to bacterial infections.

Viral growths: Relatively harmless viral diseases, such as lymphocystis and carp pox, manifest as white-pale pink lumps or waxy growths on skin and fins. There is no chemical cure but, in many cases, the fish’s immune system eventually overpowers the virus, causing the lumps to disappear in time.

Gyrodactylus

Genus of stumpy, worm-like parasites known commonly as skin flukes. Gyrodactylus rarely exceed 1mm/0.04” so are undetectable to the naked eye. They parasitise the surface of the fish’s skin and sometimes the gills where they feed and breed, giving birth to live young.

There are numerous species of freshwater and marine gyrodactylus, each with its specific range of fish hosts, and they possess sharp hooks to help them attach to fish and abrade skin.

Affected fish may repeatedly rub their bodies against submerged objects, such as rocks and plant stems, to ease the irritation.

Fish also rub in response to skin parasitic protozoa, so microscopic examination may be needed to confirm whether the problem is due to skin flukes or protozoa. Heavy infestations are life threatening and the fluke-damaged skin is prone to secondary bacterial and fungal infections. Gyrodactylus outbreaks can be treated using a proprietary fluke remedy added to the water.

Haemorrhaging

Condition in which blood escapes from the blood vessels. This can arise if the vessels become leaky, possibly due to infection, or physically rupture as a result of injury.

Skin haemorrhages appear as areas of bruising and a bacterial or viral infection is often to blame. Where bacteria are suspected, treat with a bacteria remedy. Antibiotics may also be required. Viral haemorrhaging is untreatable.

A lack of vitamins A, B1, B2, or C can lead to haemorrhaging, but this is a less common cause.

Herpes viruses

Group of viruses. Some cause disease in fish, examples being Koi Herpes Virus (KHV) and the Fish Pox (Carp Pox) virus. Herpes viruses vary in pathology and there are no chemical treatments for viral infections.

Hexamita

Genus of protozoan (single-celled) parasites. Some species infect the intestines and other internal organs. Hexamita has traditionally been implicated in Hole-in-the head syndrome (HIH; see later entry) that affects certain cichlids, but it now seems that Spironucleus, a closely related protozoan, is the true culprit.

Hoferellus

Genus of spore-forming protozoan parasites. One species, Hoferellus carassii, is responsible for kidney enlargement disease in goldfish. Affected fish develop abdominal swelling, often more pronounced on one side (unilateral). This condition may be confused with dropsy in which the swelling is generally more even (bilateral). There may be accompanying loss of balance and listlessness. It is potentially fatal and there’s no established treatment.

Hole-in-the-head syndrome

Potentially fatal condition affecting Discus, Angelfishes, and certain other South American cichlids. In affected fish, the sensory pores of the head and/or lateral line may become enlarged and pus-filled. The fish may turn dark and lose appetite, and their faeces may be white and stringy. The underlying cause(s) are unclear.

HIH is commonly attributed to infection by flagellate protozoa (notably Spironucleus species). Other factors implicated include certain bacterial infections, nutritional deficiencies and lowered immunity.

Over-the-counter remedies are ineffective against Spironucleus. The drug of choice for this and HIH is metronidazole, obtainable on veterinary prescription. It is given as a bath and/or mixed with the food. Metronidazole also has anti-bacterial activity.

Use Di-metronidazole in stubborn cases where metronidazole appears ineffective.

Helminth infections

Helminths are primitive worm-like organisms. Some species are free living, but most are parasites of various animals, including fish. Examples are various flukes, tapeworms and nematode worms that can cause disease in aquarium and pond fish.

Some helminths attach to the fish’s body surfaces, notable examples being the skin and gill flukes, while others, such as tapeworms and eye flukes, reside within the body. Helminths harm in many ways. For example, their potentially life-threatening skin and gill flukes possess sharp hooks and suckers that help them grip the fish.

These physically damage the fish’s skin or gills and render them vulnerable to secondary infections, such as those caused by bacteria.

Generally less serious are the ribbon-like tapeworms that live within the fish’s gut or body cavity. Nevertheless, a heavy tapeworm infestation can debilitate and weaken the fish.

Some helminths have complex life cycles in which the parasite must sequentially infect different types of animal host (a bird, then an aquatic snail, then a fish) in order to complete one generation.

Many ecto-parasitic helminths (skin and gill flukes) can be treated using commercial over-the-counter remedies.

Eradication of internal helminths (gut tapeworms) generally requires drugs known as anthelmintics, examples being praziquantel, mebendazole and fenbendazole. Many anthelmintics are obtainable only on prescription from a vet.

Breaking the helminth’s lifecycle, such as eliminating aquatic snails that may harbour the parasite’s larval stages, is another control strategy.

Hydrogen peroxide (H202)

Clear liquid with powerful oxidising activity. Sometimes used to treat skin parasites. Potentially harmful to fish, so use with caution and never overdose.

Can also be used in emergency to temporarily increase oxygen levels in aquariums suffering from low dissolved oxygen. Add 0.1-0.25ml of 3% hydrogen peroxide solution per litre of water for this.

Hyperaemia

Abnormal condition characterised by excess of blood in a part of the body. Hyperaemia is particularly noticeable on the fins of affected fish where it manifests as enlarged blood vessels resembling thin red streaks. It can also appear as patches of red-brown on the fins.

One possible cause is stress, perhaps due to deteriorating water conditions or overcrowding. In such cases, an improvement in environmental conditions may lead to hyperaemia clearing by itself.  

Another cause is blood poisoning (septicaemia), often due to a serious bacterial infection which will require antibiotic treatment.

Hyperplasia

Abnormal increase in the number of cells within a tissue or organ. Hyperplasia often occurs in response to an irritation or infection: for example, the fish’s gills may undergo hyperplasia as a result of damage caused by parasites such as gill flukes.     

Hypoxia

Reduced oxygen level within the tissues. Affected fish may show signs of breathing problems, such as gasping and/or fast gill beats.

Hypoxia can be the result of a gill or blood problem: for example, gill damage caused by exposure to high ammonia levels, gill parasites or nitrite poisoning of the blood.  

In some cases it is the result of insufficient oxygen in the water — environmental hypoxia, perhaps due to overcrowding or inadequate aeration. Hypoxia is potentially fatal and can cause sudden mass mortalities among fish. Methods for control and treatment depend on the underlying cause.  

Ichthyobodo

Genus of protozoan parasites, formerly known as Costia. The important species is Ichthyobodo necator. These single-celled parasites are ten microns in size (one equals one-thousandth of a millimetre) and affect the skin and gills of fish, and particularly target stressed individuals.

They are more commonly encountered in pond fish.  

Affected fish produce excess mucus, manifesting as areas of whitish, hazy skin and there may be red areas on the skin and fins, and pale gills. The fish may also develop breathing problems and exhibit repeated body rubbing ('flashing').  

When viewed under a high-powered microscope, Ichthyobodo appears kidney-shaped with two whip-like filaments known as flagellae. Treat the whole pond with a proprietary ecto-parasite remedy.

Ichthyophthirius

Genus name for the whitespot parasite and currently there’s only one species, Ichthyophthirius multifiliis, which affects freshwater fishes.  These parasites appear as small white spots beneath the outer skin and other body surfaces of the fish — hence the common name whitespot.

Immune suppression

Condition in which the fish’s immune system is weakened or impaired, making it less able to fight off infections.  

There are many causes. In pond fish, such as Koi and goldfish, seasonally low water temperatures slow down the fish’s immune system. This explains why over-wintering pond fish are more prone to opportunistic infections such as fungus and carp pox virus.

Fish stressed, perhaps due to overcrowding or poor water conditions, can also become immuno-suppressed. This is because stressed fish produce elevated levels of certain hormones such as cortisol which has a direct inhibitory effect on the immune system. Old age, poor nutrition and certain medications (for example, the antibiotic oxytetracycline) can also suppress immunity.

Infertility

Condition in which the fish is unable to produce viable eggs or sperm (milt). Infertility can have many causes. There may be natural reasons; the fish can be sexually immature, is too young to breed, or it may have an internal tumour or disease that has damaged the reproductive organs.

Poor nutrition and genetic defects have also been linked to cases of infertility. Some cultured fish are intentionally rendered infertile (sterile) to prevent spawning-related aggression or to increase growth performance — as in the case of some food-fish species. The underlying cause of infertility is often difficult to determine.

Iodophors

Iodine-based disinfectants. Effective against bacteria, viruses and fungi. Iodophors are used in some commercial aquaculture facilities to disinfect equipment — and fish eggs — but should never be exposed to live fish as these chemicals are highly toxic.

Kanamycin

Antibiotic. Fairly stable in water so can be administered as a long-term bath treatment. Can also be given orally with the food. Use with caution as kanamycin may be toxic to some fish species. Seek professional advice if in doubt.

Khawia

Genus of tapeworms. One species, Khawia sinensis, (originating from Asia) is sometimes found in the intestines of common carp and Koi in the UK. The tapeworm’s larval stage is carried by aquatic annelid worms, such as Tubifex. Khawia infestations are treatable with special drugs known as anthelmintics.

Tapeworms will be described in their own A-Z entry.

Latent infection

A dormant or ‘hidden’ infection. Fish harbouring a latent infection may appear perfectly normal and healthy, hence the infection often going undetected. Several types of virus, including the carp pox virus, may remain latent in their fish host for many months, possibly years. Typically, only when the fish becomes stressed and/or its immunity is lowered does the infection reawaken, causing disease symptoms to manifest.

Lernaea

Genus of parasitic crustaceans, commonly known as anchor worms which parasitise the skin and gills of fish.

Lernaea are more commonly found on pond fish, but occasionally occur on aquarium fish, especially wild caught specimens.

Only the females are parasitic, reaching 5-10mm/0.4” in length and resembling elongated strands attached to the fish’s skin. Each female produces a pair of egg sacs that give her a spindly Y-shaped appearance.

In their juvenile stages lernaea attach to the fish’s gills. Parasite damage to the skin renders the fish prone to bacterial infections and gill damage may cause respiratory problems.  

Adult lernaea can be carefully removed using tweezers, but special anti-crustacean chemicals may be needed to completely eliminate anchor worms  the pond.  

Lepidorthosis

Abnormal erection of the fish’s scales. This typically accompanies the bloating condition known as dropsy.    

Lethargy

State of drowsiness or lack of energy. This is often caused by stress, disease, or adverse water conditions, so should be investigated without delay.  Exposure to low water temperatures (as experienced, as an example, by over-wintering pond fish) can also cause a lethargic or torpid state. Bear in mind that many fish become lethargic when 'sleeping' — and this is normal!   

Levamisole

Drug for treating gut-dwelling nematode worms, such as Capillaria worms that affect Discus, Angelfish and other cichlids.  Available as Levamisole hydrochloride which can be given orally — incorporated with food — or added to the water at 1-2 mg/l as a 24-hour bath.  

Lordosis

Abnormal curvature of the fish’s backbone which becomes kinked in the vertical plane.  Typically, lordosis develops slowly and there are many causes, including genetic or developmental defects.  

In some cases, a chronic bacterial infection, such as caused by mycobacteria, may be responsible.  Various dietary deficiencies, including a lack of vitamin C (ascorbic acid), may lead to this condition.

Backbone deformities can also occur in old age.  

Where lordosis occurs suddenly (within 24 hours) then a physical injury to the backbone is the most likely cause.

There is no known cure for this condition.

Lymphocystis disease

A relatively harmless viral disease that affects certain freshwater and marine fish. It manifests as white to pale pink wart-like growths on the skin and/or fins.

There can be single or several growths that sometimes develop into cauliflower-like clusters.

Affected fish typically remain otherwise healthy and do not body-rub — in contrast to fish suffering from whitespot or other irritating skin parasites. Often, only a single fish is affected.

Lymphocystis affects the more evolutionary advanced groups of fish such as cichlids, anabantoids (Gouramis and relatives) and Glassfishes. It does not affect catfishes, barbs or tetras.  

There’s no chemical treatment, but recuperate affected fish under optimal conditions. This will help the fish’s immune system to overpower the virus, causing the growths to eventually disappear.  

The disease generally attacks fish already stressed or diseased, so investigate why they became susceptible in the first place.

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