Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish


Argulus on Stickleback
Argulus on Stickleback
Photo by Michal Grabowski - Wikipedia


Argulus are parasites which attach themselves to fish and then bore in to the fish's flesh and feed off the fish's blood. Obviously this weakens the fish and a heavy infestation will quickly kill the fish. Argulus are quite rare in the aquarium and an infection is likely to be caused by a solitary argulus which has sneaked in from a fish farm, introduced accidentally with some live food or on wild caught fish . Argulus infections in ponds is another matter entirely. In ideal conditions where there are lots of potential hosts and easy living the argulus can reproduce very quickly and become well established making treatment that much more difficult.


Argulus are often referred to as fish louse or fish lice. They are crustacean ectoparasites (external parasites). There are three species which can be found in the UK and all are equally damaging to fish.

Argulus will begin egg laying when the water temperature reaches 10C or more and will continue until the water temp drops to below this level. In ponds they will reach their maximum numbers towards the end of summer.

Argulus are small flat disc shaped in appearance and a greenish to amber translucent colour and when attached to large goldfish or koi they can be very difficult to see.

Argulus feed by injecting the fish with a venom from a spine, the venom begins digestion of the flesh and then the argulus uses it's mouth parts to ingest the pre digested flesh. Argulus are very mobile and do move from fish to fish, this obviously risks the spread of disease and the wounds left by argulus are an easy route for secondary infections to take a hold.


The old fashioned treatment was to use potassium permanganate and although quite effective it does have several disadvantages.

  • It oxidises quite quickly in water especially if the water is full of organic compounds as ponds and aquariums are likely to be.

  • It is quite a toxic compound to fish and prolonged treatment is likely to be very stressful to fish.

  • Some infestations may not be completely destroyed by it which means the whole process will restart within weeks of treatment.

  • To remove a heavy infestation from a fish use a dose of 1g per 10 litres of clean water in a bath. This will be easier and more effective than trying to remove them manually and although this is extremely effective against juvenile and adult argulus with 100% mortality it will not solve the problem in the environment. and in tests using a much stronger solution of 1g per litre failed to kill the eggs of argulus.

  • It will kill all the beneficial filter bacteria.


Organophosphates used to be a recommended method of treating argulus. as effective as they were as a treatment there use is strictly controlled and are hazardous to both fish and human health. Because of the risks to health they pose organophosphates are now banned in the UK as a fish treatment.

Other treatments

Other common broad spectrum treatment such as salt, formalin and malachite green are all relatively ineffective and not recommended.

Commercial remedies

There are several safe and effective commercial remedies available in the UK. Using one of these should be the first course of action.
I have used Waterlife Parazin myself and found it to be 100% effective, but there are others worth trying too.

Treatment Finder

References - Natural history museum.
Inter Research - Diseases of aquatic organisms.