Fish Pests and Diseases

Fish Pests and Diseases

There are many pests that can afflict fish, but the majority are readily controlled. Most outbreaks are caused by stress in the fish and rarely do any infestations simply break out in the well ordered pool. Fish can also suffer from a number of debilitating diseases, which can look quite frightening, but they rarely appear in the well managed water garden. Many infections in ponds break out as a result of the fish being stressed due to poor water quality, so check the water quality as a matter of course.



Fish Pests and Diseases - anchor worm This is a very tiresome and unpleasant crustacean that is a common parasite of the goldfish and carp family. It is a small creature, rarely more than 6mm (1/4in) long, with a slender tubular body and a barbed head, which it embeds in the flesh of its host, causing unsightly lesions and tumour-like growths. It often looks particularly hideous, as it develops a light covering of green algal growth.

Treatment: The most practical method of dealing with anchor worm is to capture each infested fish, hold it in a wet cloth and dab the parasite with a 0.1 per cent solution of potassium permanganate, using a small paint-brush. This will kill the creature, which can then be gently withdrawn with tweezers. Finally, treat the wound with a proprietary fungicide. There are also several proprietary parasite cures, which can be added to the water of the pool to destroy anchor worms at the free swimming stage of their life cycle.



Young fish may be devoured by the various diving beetles and their larvae, or naiads. The beetles vary in size and colour, but the great diving beetle (Dytiscus marginalis), which is about 5cm (2in) long and has a yellow-bordered, dark brown body, is the commonest. Most pool owners tolerate the activities of diving beetles, regarding them as part of the natural fauna of the water garden and living with the minimal destruction they cause.



Some water gardeners are concerned about dragonfly attacks on young fish, for these vicious predators are killers. Although adult dragonflies are beautiful to watch, their larvae, or naiads, which spend up to five years living and eventually pupating in the pool, are very destructive, posing a threat particularly to young fish. Rarely is a fish killed in one go, but serious injury can be inflicted. The naiads live among submerged aquatic foliage. As soon as suitable prey comes into view, this keen-eyed predator will shoot forward its strange face mask. This is like a pair of hooked jaws, which grip the prey and bring it back to the gaping mouth.

Treatment: There really is no satisfactory way of dealing with this problem. As the adult stage of the life cycle is so lovely, and most pond owners enjoy the presence of dragonflies flitting above the water, the unfortunate dietary habits of their young are best tolerated.



Fish that are suffering from this disorder become distended, the scales standing out from their body and resembling the needles of a pine cone. This can be caused by an internal bacterial infection, a kidney disorder, or an influx of water.

Treatment: Controlling dropsy is possible if the cause of the swelling is known. Placing the fish in a salt bath (25g/1oz) rock salt per 4.5 litres/1 gallon of water) for seven days will often reduce the swelling. Treating with an antibacterial remedy can also be effective if bacteria are responsible. The best remedies are those sold for treating ulcers, as these are caused by the same types of bacteria.



Caused in most cases by a combination of bacterial organisms, this infection usually begins on the edge of the tail or fin, which slowly rots away until nothing but a stub remains. The first indication of trouble is a distinctive white line running along the outer margin of the fin, which gradually progresses downwards to leave the outer margin badly frayed, the soft tissue between the hard rays of the fin disintegrating.

Treatment: If the disease is caught early enough, a proprietary fin rot cure can he used. In most cases, the fin tissue will heal satisfactorily and regeneration will take place. Vhere the disease is so bad that the rot has reached the base of the hard rays and is beginning to invade the main body tissue, the use of antibiotics may be necessary



There are several species of fish louse, which cause distress to pond fish. They are parasitic crustaceans that cling to the bodies of fish to feed and then drop off to digest their blood meal. Although some have specific host requirements, they all look much the same, having a strange flattened circular carapace with a diameter of up to 1cm (3/8in) and feelers.

Treatment: There are specific proprietary parasite remedies, containing the active ingredient dimethyl-trichlorohydroxyethl phosphonate, which can be used to control this parasite. However, in some countries, including the UK, these are only available from a veterinarian. Such treatments are harmful to orfe and rudd. It is important to treat the whole pond to control the parasites that are not on the fish.



The various fungal diseases that afflict fish usually appear as a downy cotton-wool-like growth on the fins, tail or body. They normally occur as a secondary infection on wounds caused by the fish colliding with a sharp object or rough handling by the pool owner, as a result of the fish becoming run’.s down during a mild winter, following attacks by pests such as anchor worm or white spot, or due to poor water quality. There is also an unrelated ‘fungus’, which is caused by a bacterium and referred to commonly as mouth fungus. This appears around the mouth as small cotton-wool-like growths that eventually destroy the tissue.

Treatment: The true fungal diseases are easily treated, provided they are not allowed to become deep seated and widespread. Proprietary fungus cures are very effective when added to the pool. Sometimes, several treatments may be needed, but they are generally very effective. A salt bath, (25g/1oz rock salt per 4.5 litres/1 gallon of water) is the traditional method of coping with fungus. However, although effective, it is a long-term process and, in severe cases of the disease, usually does not act quickly enough. Because fungal disease normally occurs as a secondary infection, it is important to identify and correct its primary cause to prevent recurrence soon after treatment is complete.

Provided that it is identified in the early stages, mouth fungus can be controlled effectively using a commercially available bacterial treatment.



These are minute creatures that infest the gills and bodies of decorative pond fish, causing them to swim about haphazardly or gasp at the water surface. They rush around the pool, banging themselves against the sides and rising to the surface in sudden fits. The rate of respiration increases, while the fins constantly twitch.

Treatment: Use one of the many good parasite cures on the market. Ensuring that the water is well oxygenated during treatment will ensure that the fish survive while damaged gill tissue regenerates.



These are well-known predators of creatures of all kinds, and fish have their very own species. In common with other bloodsucking leeches, they have a sucker at both ends of the body, which are powerful attachment organs. They also have sharp, cutting mouthparts, and secrete an anti-coagulant to prevent the blood from clotting. A single leeche can, in a single meal, take in ten times its own weight of blood. This can last for a very long time and be very distressing for the fish. Before treating the fish, check that the leech is actually attacking it; most species depend on snails and similar creatures for their nourishment.

Treatment: Removal of leeches is simple: immerse troubled fish in a salt bath, such as that traditionally employed for treating fungal diseases (25g/1oz per 4.5 litres/1 gallon of water). The leeches usually drop off after this treatment. Never try to remove them without treating them first, as they will create very unpleasant wounds if withdrawn forcibly. Apply a proprietary fungus cure after the leeches have been removed to protect against secondary infection. Unfortunately, this does not control any leeches that are not actually attached to the fish. Therefore, several treatments are usually necessary.



This disorder causes the fish to become sluggish and rise to the surface of the water. They show reddish-brown on their sides and bellies, and occasionally on their anal or pelvic fins. If left untreated, it will spread throughout the pool. It is a bacterial infection usually associated with overcrowding or poor water quality.

Treatment: If caught in the early stages, improving the water quality through a partial water change, followed by treatment with a general external parasitic remedy will be effective. More advanced cases require antibacterial or antibiotic treatment. This may also be the initial stage of ulcers and should be treated with an ulcer remedy.



Ulcerated patches appear on the bodies of affected fish in a random manner, exposing raw tissue that may be streaked with blood. It is a distressing disorder for the fish and for the pond owner to witness. In the main, it appears to be caused by the same, and related, bacteria that cause fin and tail rot. Ulcers often occur in fish that are weakened or stressed in some way. Therefore, it is important to identify and correct the primary cause, as well as the ulcers themselves.

Treatment: Proprietary treatments are available that can control ulceration in its early stages. In more advanced stages, it is necessary to apply the remedy directly onto the wound, or to use antibiotics, either in a medicated food or, for larger fish, by injection.



Badly infested fish look as if they have white measles, take on a pinched appearance and swim in a drunken fashion. If left untreated, the fish usually die. This is a widespread parasitic disease that usually appears in freshly introduced or stressed fish. High stocking levels in the pool wil increase the likelihood of transmission of these parasites. Vhite spot disease is caused by a member of the group of single-celled creatures known as protozoa. This is tiny, scarcely 1mm (1/24in) across, and spends part of its life cycle embedded in the skin of fish, causing extreme discomfort and often death.

Treatment: Fortunately, white spot is easily cured using proprietary treatments. These must be introduced to the pool to control the white spot parasite in its free swimming stage, as well as on the fish. The treatment may take effect in anything from three to 14 days, taking longer at low temperatures.



These are often referred to as diseases, although in fact, they are either caused by tiny, single-celled organisms, known variously as costia, cyclochaete and chilodonella, or by poor water quality. Affected fish often swim on the bottom of the pool with their fins folded and rub against underwater objects. Their bodies become covered with a bluish-white deposit — a mixture of the parasites and an excess of slime.

Treatment: A proprietary parasite cures will usually be effective. Combine treatment with improved water quality.


19. March 2011 by admin
Categories: Gardening Ideas, Water Gardening/Water Features | Tags: | Comments Off on Fish Pests and Diseases


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