Parasites - Costia

The Costia fish parasite is another microscopic organism. The malicious Costia parasite on koi fish is extremely small and quite difficult to diagnose due to its size. It is basically impossible to determine the presence of Costia parasite on koi fish without the aid of a microscope. Costia is comparable in size to a red blood cell but its erratic spinning motion is unmistakable.

Costia is shaped like a bean or comma. It has two long uneven flagella, or hair-like appendages which are used for mobility. The Costia parasite on koi fish will feed on its host by spearing the cells of the skin and gills and sucking out the content. In response to the vicious parasitic attack koi will become lethargic and may rest on the pond bottom. Also, the koi may increase the amount of mucus on its skin which can make the area appear slightly grayish. As with many other parasites damage done to the host fish by Costia will enable bacteria to invade the skin and encourage an outbreak of skin lesions and ulcers.

Water temperatures from 39⁰-85⁰F are ideal for Costia, though it really prefers the lower end of this range. Every Costia parasite must be attached to a host to sustain life; it dies within a matter of hours if it becomes free-swimming.

What are koi Costia symptoms?

Look for Costia in koi fish that exhibit these signs: Lethargy, redness or irritated skin, excessive mucus covering on skin, isolation and weakness.

What are the symptoms of Trichodina parasite in koi?

Work In Progress


Costia has various options of treatment. They are in order of the best method.

Malachite Green (MG) and Formalin (F) Not to be used below 13̊C MG 2% 10ml per 176 UK gallons F 30% at 10ml per 140/150 UK gallons

• Salt Treatment (Do Not Use Other Treatments While Salt Is Present)

For fish that have gill damage and/or difficulty breathing, salt baths are the safest option. This salt bath should last for 72 hours to be sure all costia are dead. To minimize stress to your fish, first start with a concentration of 3 grams per liter (.3%). After 12-24 hours, increase this another 3 grams up to .6%).

If your fish seem to be handling this alright and don’t appear stressed, increase again to a maximum of .9% after 48 hours. Fish should be eating normally again and shouldn’t display signs of discomfort like flashing or clamping fins. You can now slowly decrease salt levels by performing daily water changes of 25%.

• Potassium Permanganate

Pond treatment for parasites and snails: 2-4 g / 1000 liters.
Keep on adding in small doses of 2g/1000 litre to keep the pond water pink for 1-2 hours

Pond treatment for bacteria and ulcers: 2-4 g /1000 liter
Keep on adding in small doses 2g/1000 litre to keep the pond water pink for 8-10 hours

Dip:  very effective for parasites, bacteria, ulcerations etc. Use 1 gram per 10 liter water for 7 minutes. Always aerate and ALWAYS remove the fish if it gets restless and jumpy. Control timing precisely and do not treat too many fish at the same time unless you can time the treatment of each fish correctly.

This treatment tough, can push an ailing fish ‘over the edge’. Therefore fish already weakened by disease should first be treated with a 30-50% dose first time. Alternatively simply shorten you dipping to 1-2 minutes if the gills are infected and necrotic.

Important. Always de-clog the gills with a Peroxide (hydrogen peroxide) treatment to the pond. It not only terminate the action of the potassium permanganate, but will clear the water from the brown colour. Will also aid in removing the brown coloured necrotic tissue from the gills.



I want to warn about use with salt. Salt (NaCl) is additive to potassium permanganate. To strip parasites from basically HEALTHY FISH can be tolerated, but sick, weakened fish should never be subjected to such treatment and will surely die. Always test salt level or ask the pond owner about his salt regimen before deciding on the protocol for the potassium permanganate treatment.

For certain phenotypes of the Doitsu koi the uptake of oxygen is poorer for certain physiological reasons. They stress more quickly and die due to oxygen starvation. Use half the recommended dose/time when dipping the first time at least to determine their suitability for the treatment.

Any treatments added are done so at your own risk. It is your responsibility to know your pond volume and to calculate dosages correctly. 

Golden Rules Of Treating Koi

Scrape the Koi first before treatment

Identify which Parasite is causing the problem before treatment

Make sure the pond water is salt-free because it can harm Koi when used with most chemicals

> Calculate the Volume of Water accurately

Turn off the UV Unit when treating your Pond

Test your Water Parameters before Treatment

> Follow the label’s instructions and do not overdose. Chemicals can cause serious damage to your Koi if amounts are miscalculated