Worms in tropical fish

Tropical fish can be infested by a variety of parasites, and getting an exact diagnosis can sometimes be a bit of a challenge. Intestinal worms can be divided into 2 main groups, roundworms and tapeworms.


Roundworms, or nematodes, are internal parasites which live in the digestive system of fish. Although fish can carry small numbers of roundworms without any apparent ill effects, they can cause a variety of problems including loss of weight and even in severe cases they can kill fish. The main problem worm species are Capillaria and Camallanus.

Capillaria Species

Life Cycle

Capillaria worms can infest a wide range of fish species including cichlids (Discus and Angelfish are commonly infected), carp, goldfish, gouramis and tetras. The worm life cycle is simple, and does not require any intermediate host. The adult female worms lay eggs which can take up to 3 weeks to develop depending on the water temperature. When the embryos hatch from the eggs, they can be eaten by susceptible fish and then the worms will develop in the fish, with the adult egg-laying stage being reached in about 3 months. However, sometimes the Capillaria worms can inhabit ordinary (tubifex) worms and then the fish can become infected by simply eating the infected worm.


Once inside the fish, the parasitic worm develops in the intestinal tract of the fish and can cause symptoms ranging from weight loss to intestinal blockage in heavy infestations and even death.


Definitive diagnosis is tricky in live fish – post-mortem findings can often demonstrate the presence of the adult females which can be identified by the brown barrel-shaped eggs within them, or even in some cases the eggs themselves can be seen in the intestinal tract of the fish. Immature larvae or adult males can occasionally be missed during post-mortem examination since they are almost transparent.


The worms can be simply treated by the administration of Levamisole to the pond or tank water containing the affected fish. Also good hygiene will help – remove any organic debris or faeces as often as possible following treatment.

Take care with feeding the fish and be aware that certain types of live food may contain these nematodes and could re-infect your fish.

Camallanus Species

Life Cycle

These worms infect freshwater fish such as guppies, swordtails, cichlids and other freshwater species. They reproduce by producing live offspring since the females incubate the eggs while they are in their bodies to produce the larvae. These larvae are passed out in the faeces where they are ingested by other creatures such as copepods. This means that the worm life cycle is ‘indirect’ – in other words they require to infect another host and partially develop in order to be able to infect fish. The stage of development within the copepod is that of second stage larva developing into third stage larva, and when the infected copepod is eaten by a susceptible fish the third stage larvae are released into the fish where they finally develop into adults, and the whole reproductive cycle begins again.


One of the first symptoms which may become evident is a reddish coloured worm protruding from the anal vent of an affected fish. Affected fish also lose weight and can become more susceptible to other diseases.


In the absence of any visible external symptoms, diagnosis can be confirmed by post-mortem examination of the gastro-intestinal tract where the characteristic worms can be found. If any females are present it is often possible to find them with both eggs and first stage larvae within them.


These worms can be readily treated by the administration of Levamisole to the tank water, and since they are confined to the gastro-intestinal tract treatment is usually very successful. Also since copepods or similar crustaceans are required for the successful transmission of this parasite, take care not to feed them to your fish as part of their live feed.



These parasites, also called cestodes, are internal parasites that affect a wide variety of fish species including Carp, as well as aquarium fish such as Discus. There are a number of different tapeworms, but all of them have the same ultimate effect on the fish which they infest; they affect the digestive system and can eventually cause blockages. Any tapeworm infestation will affect digestion, and as a result the body weight and general health of the fish will be affected too.

Tapeworms have an indirect life cycle, that is they require an intermediate host in order to complete their life cycle. These intermediate hosts can be copepods, or other small creatures. The route by which fish become infested with tapeworms is usually subsequent to the introduction of infected fish. Once a tank is infected, the only way the infestation can be completely cleared is by removal of the intermediate hosts when the fish are treated. However, this is not usually an option with established tank systems, so the next best solution is to routinely treat the fish with Fluke-Solve® Aquarium on an annual basis to ensure that there is no build-up of tapeworms in the fish. Also if symptoms are seen such as stringy faeces along with weight loss then the fish should be treated right away with Fluke-Solve® Aquarium.

As a general disease control measure, any new fish should be treated with Fluke-Solve® Aquarium during quarantine especially before putting them into the tank, preventing the introduction of this problem.


Before you anaesthetise a fish you should check the fish using the checklist below:

- Fish should be viewed in the water (from above and from the side if possible), assessing their movements and respiratory rate. In general, fish movements are fluid and seem relatively effortless.
- The respiratory movements are generally slow and not obvious. Wooden movements, or marked respiratory movements, should be viewed with suspicion.
- The skin in general should be smooth and unbroken; haemorrhages may suggest localised scale damage or more serious septicaemic problems.
- The scales should normally lay flat against the body. Lifting of scales to produce a ‘pine-cone’ effect may be due to localised or generalised fluid build up in the skin, or due to abdominal swelling due to ascites (fluid in the belly of the fish).
- During the overall assessment of the condition of the fish including observation of its respiratory action, gills may be glimpsed partially. In the majority of cases it will be necessary to lightly anaesthetise fish to examine the gills in detail. The gill can be seen by gently lifting the operculum and using a suitable light source to illuminate the buccal cavity. The gills should normally be a healthy ‘salmon pink’ colour, with clearly demarcated primary lamellae.


Anaesthetising fish is fairly easy but still isn’t something to be done lightly. Any anaesthetic carries a risk, sometimes because the fish has an unknown/undiagnosed problem which can cause an issue. For this reason we don't suggest the routine use of anaesthesia of koi carp just to give them a general check-up. However there are instances where anaesthesia is extremely useful, such as:

- surgery/debridement
- handling of valuable fish (particularly large fish)
- close examination of a fish with a problem


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