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Pond Fish Disease

Author: Bernice Brewster

Keeping fish in the garden pond is a truly relaxing hobby but if something goes wrong and the fish start to get sick, it can be extremely distressing. It is always worth e-mailing for advice when any of the fish become sick as it may simply require the addition of one of the range of proprietary medications to the pond to solve the problem. For any of the medications to be successful, it is important to have identified the reason why the fish have become sick. In the event of an outbreak of disease being caused through poor water conditions, it is important to improve the water quality before embarking on a course of treatment. There are some occasions when it may be necessary to seek the help of your local Vet to cure the problem, such as in the outbreak of a persistent bacterial infection.


How do I know when the fish are sick?

As the water gets progressively warmer through the spring and into the summer, the fish should become increasingly active, swimming with fins erect and seeking food. In the summer the fish will actively feed two or three times a day and during this period it is very easy to monitor the behaviour and ensure they are all active and eating. The first sign of a sick fish in the warmer months of the year is when it shows little or no interest in feeding and often separates itself from the other fish in the pond.


In the winter, when the water temperatures are cold, it is perfectly normal for the fish to cease feeding altogether and lie very quietly on the bottom of the pond and only moving occasionally. Generally, the signs which indicate a fish is sick are: colour changes, either becoming significantly paler or darker, repeatedly rubbing or jumping, separating itself from the others, inactivity, fins clamped against the sides of the body, loss of appetite, getting progressively thinner, hanging near a water inlet or gasping at the water surface. Clues as to the cause of the problem can be obtained from the rate of onset: All species and sizes of fish are affected and occurs very rapidly - suggests a water quality problem. One or two fish are affected but the number gradually increases over a period of a week or more and only one species of fish appears to be suffering - suggests an infectious disease. Only one or two fish are affected, which are not necessarily of the same species and it does not spread to any other fish in the pond - suggests either a physical injury or even a deformity.

How does the water quality make fish sick?

In the event of a fish becoming sick, or even dying, it is always worth testing the water, if only to eliminate this as the cause of the problem. has the complete range of test kits suitable for monitoring all pond water conditions from pH to dissolved oxygen, ammonia, nitrite, nitrate and alkalinity. Fish produce nitrogenous waste in the form of ammonia, which in an established pond is either utilised directly by the submergent plant life or is broken down by the filtration system to nitrite and then nitrate through the activity of bacteria and other micro-organisms. Both ammonia and nitrite are extremely poisonous to fish, even in very small quantities, whereas nitrate is not thought to be particularly harmful in low concentrations. Ammonia affects the gills, causing the cells of this delicate tissue to grow abnormally which causes the individual filaments to become fused together. This in turn affects the fish’s ability to breathe as well as get rid of the harmful ammonia and regulate the water content of the body. As ammonia accumulates in the environment, it also builds up inside the body of the fish, affecting the way the blood and tissues work. Nitrite is a skin irritant, which causes the fish to rub and jump, which might also be mistaken for the effects of parasite infection. This breakdown product of ammonia has a more sinister effect on the blood of the fish, by binding very tightly with the red pigment in the blood cells and preventing it from carrying vital oxygen. Nitrite also relaxes smooth muscle, which can lead to the collapse of the heart and circulatory system. There is also an accumulation of evidence, which suggests that nitrite can affect the immune system and make it less effective. Poor water quality causes the fish to become stressed and as a consequence suffer a range of diseases, such as fin rot and white spot. In the event of discovering the pond has a water quality problem such as caused by ammonia or nitrite, partial water changes should be undertaken.


When topping up the pond, a water conditioner should be used to remove chlorine and other harmful additives from the tap water. have in stock a wide range of tap water conditioners for use when topping up the garden pond.

What are the common diseases which affect pond fish?

There are a number of diseases, which are commonly encountered, even as a consequence of one or more stress related conditions. Most of these common diseases will respond rapidly to treatment by one of the proprietary brands of medication.



White Spot (Ichthyophthirius multifilis)

This parasite probably kills more fish worldwide than any other true fish disease. Infected fish tend to become lethargic, fins clamped, lose their appetite and repeatedly rub on objects in the pond. Examination of the affected fish shows them to have tiny white pimples on the body and fins, including the eyes and gills. Some male members of the carp family develop a spawning rash during the breeding period, which can resemble white spot. A spawning rash is characterised by regularly arranged pimples around the gill covers, snout and bony fin rays, it is never on the eyes or gills. Fish in breeding condition tend to be bright and alert, unlike sick fish infected with white spot. Control of white spot requires the pond to be treated as the medication kills the free-living stage of the parasite. There is a range of treatments suitable for controlling this parasite, readily available at Prestwood PetZone.



Slimy skin or a film over the skin

The fish tend to rub and jump, clamp their fins against the body and sometimes become listless and cease feeding. Examination of the fish shows it has a grey film over the body surface, making it appear dull. This grey film is usually excess mucus that the fish has produced in response to an irritant, such as poor water quality or even chlorine from the tap water as well as a host of microscopic parasites such as Costia, Chilodonella, Trichodina or skin and gill flukes. Test the water to eliminate this as the cause of the problem. Most of the microscopic parasites will respond to treatment of the pond with one of the many proprietary anti-parasite medications.


Fungus appears on the fish as a cotton wool like growth usually a brown colour as it traps pieces of dirt and detritus in the fibres. It is quite remarkable the speed at which a fungal infection will spread over the body of an infected fish. Fish tend to become susceptible to infection when the skin is damaged and this may be due to some environmental pollutant such as ammonia or nitrite. There are a range of medications available for controlling fungus in the garden pond but probably one of the most effective is a pond salt. The fungus cannot grow in a mildly saline environment but the salt can only be used if there are no plants in the pond. Aquatic and submergent plants will die if exposed to the salt. Salt does not break down and will only be removed from the pond by partial water changes. If salt is added to the pond, care should be taken if adding other medications, as some are not compatible with it.



Fin Rot

Fin rot is actually caused by bacteria, which are always associated with the fish. If the fish become debilitated through illness or stress the bacteria can increase in numbers, then erode the fins and tail, to leave the bony rays protruding at the edges. Sometimes the affected fin or fins can be obviously inflamed and the erosion can eventually affect the skin and muscle. Use a medication suitable for controlling the fin rot but the underlying cause should be identified and treated in addition to the symptoms.



Ulcers and lesions

Often the cause of a lesion or ulcer is initiated by physical damage, which becomes infected. Initially, the area may just have a few raised scales, which might be reddened and inflamed. Often these areas of inflammation heal without any need to intervene but sometimes the infection progresses, giving rise to an open ulcer, where the underlying muscle is exposed. Ulcer disease may affect only one or two fish, which rapidly recover but where increasing numbers are affected then treatment is required. There are an assortment of medications which will help with ulcer diseases by generally reducing the bacterial population in the pond. If none of these medications are effective in controlling the spread of the disease, then it is advisable to seek the professional advice of your local Vet, for a stronger medication. Keeping a pond is a very rewarding and relaxing hobby. Although there should be a general awareness of the possibilities of disease outbreaks, providing the water quality in the pond is good and there is a satisfactory dissolved oxygen content, most fish remain healthy.


For a range of products to treat pond fish disease, please have a look at our pond disease treatments page. Copyright Pets Parade Ltd. 2000

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