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Parasites in Your Fish Pond – What to Look for? And What Treatments are Available?

11 July 2016

Parasites can cause a variety of issues in your pond including serious health problems for your fish. There are a number of different types of parasite that pond keepers need to be aware. By identifying the presence of these parasites early, you can save yourself an awful lot of work and eventual heartache.

White spot

Ichthyophthirius multifiliis, or ich for short, better known as white spot, is the largest known ciliated protozoan found on fish. It will quickly cover the fish and then start to take over a whole pond if left untreated.

As the name suggests, the parasite becomes visible as a result of white spots appearing on fish which are the parasite already maturing in its life cycle. To begin with, you might notice some ‘flashing’ which is when the fish rub against objects to sooth their scales. Some fish may also become lethargic and have clamped fins close to their body.

White spot can be difficult to eradicate as the treatment will only be effective whilst the parasites are free swimming; or in other words, once the white spot has actually popped and released the next life cycle into the water. The parasite which is visible on the fish will not be affected by the treatment.

Consequently, you will find that treatment takes approximately two weeks to work, which is the length of the parasite’s life cycle. Treatment will often include repeat dosing in order to target the newly hatched parasites and prevent them from re-infecting the fish.

In most cases, it is also advised to continue treatment for one week after the spots have gone to ensure that they have been removed completely. Some of the most effective treatments on the market include malachite or green formaldehyde. NT Labs F-M-G is a combination of both treatments, but due to its strength, it is not recommended for sensitive fish. If you keep more sensitive species, it is best to only use a malachite treatment.


Flukes are trematodes, a species of parasitic flatworm. Common examples that cause problems for fish are skin fluke (gyrodactylus) or gill fluke (dactylogyrus).

The skin flukes tend to be small and circular in size, while the gill flukes tend to be long and thin. They attach themselves to the skin of fish and hook in which often causes damage to the fish that results in infection. The skin fluke is hermaphroditic and so just one can multiply very quickly, establishing a colony of 2,000 within 30 days when conditions are right. The gill fluke is less prolific as it is an egg-laying parasite, however it can still dominate a pond very quickly.

The presence of flukes also causes flashing amongst fish and they can usually be seen with clamped fins near the bottom of the pond. It is visible in the form of slightly raised opaque patches often found on the fins of fish. Excess mucus may also be secreted by the fish.

Treatment depends on the severity of the infection. For example, a single fish could be treated with salt water dip, although this will only remove parasites currently on the fish, and won’t treat further generations of the parasite that breed in the pond itself. If the outbreak is more advanced, then NT Labs Formaldehyde is an excellent treatment; however, it could be too strong for sensitive fish breeds. Acriflavin can be particularly helpful for fish that have had flukes as it will help to sooth the skin and is also a mild antiseptic so will reduce the chance of any secondary infection.

Anchor Worm

Lernaea or anchor worm parasites as they are more commonly known are clearly visible, and can be seen sticking out from under fish scales. The main head of the parasite is burrowed into the skin, and the remaining body hangs out. Similarly to flukes, secondary infection is a concern as wounds can be created as a result of this ‘anchoring’ technique.

Those fish infected with anchor worm should be quarantined to ensure that the parasite doesn’t continue to release eggs. Unfortunately the only treatment is to remove each parasite with tweezers.

Fish will need to be anaesthetised in order for this to be done successfully. The entire anchor worm must be removed to prevent infection. If you do not have experience of treating anchor worm, then it is not advisable for you to administer the treatment by yourself. By dipping the tweezer in permanganate in advance of pulling the parasite out, will often encourage it to release its hooks and enable you to successfully remove it in one go.


Argulus is a fish louse which suctions onto the skin and fins of fish. They cause serious irritation and inflammation. They appear like small flattened discs and measure up to one cm. They feed on the blood of fish and often cause infection as a result.

The most common treatment for argulus is salt dip. Waterlife Parazin P is an effective treatment if you are able to quarantine the fish in smaller volumes. However, if you have sensitive fish like rudd and orfe it would be better to treat them using salt water dip.


This microscopic parasite lives across the fish and will cause serious irritation over time as it multiplies. The first visible sign of trichodina is a milky secretion across the skin which is created by mucus as a result of the irritation. Once again, flashing will occur, but the only way to verify that trichodina is present is by a skin scrape under a microscope.

An anti-parasite treatment such as NT Labs Eradick Anti-Parasite is one of the best means of treating trichodina. This is a general base treatment that is suitable for the more sensitive species of fish as well. Once again, acriflavin can also be helpful in order to reduce the chance of secondary infection.


Costia is another microscopic parasite than can only be identified as result of a skin scrape. Small numbers of the parasite are often found in healthy fish. It is only when the parasite is present in larger numbers that fish tend to be adversely affected by its presence. Costia thrive in lower temperatures and cannot survive over 30 degrees Celsius. Like many of the aforementioned parasites, it causes irritation which results in clamped fish with milky skin.

An all-round treatment such as Blagdon Anti Parasite is one of the best treatments for costia.

Before administering any treatment it is imperative to ensure that it is suitable for the fish species you have. It is also vital to ensure that you follow the instructions carefully, and ensure you do not overdose, as this can cause more damage than good. Also remember that when using many treatments you will need to remove carbon and zeolite from your filter canister as the treatments will be removed from the water by this. You must also ensure that UV clarifiers are turned off during treatment, as they will kill the active ingredients in the treatment.

When it comes to fish disease the key is to check your fish regularly and keep an eye on their behaviour. The better understanding you have of their behaviour, the faster you will notice if something is wrong, and be able to ensure that they are treated as quickly as possible.

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