Fish Disease Diagnosis
Fish Disease Treatments
Ponds, Aquaria & Filtration


Even one parasite warrants immediate action

The parasite Chilodonella is a major threat to fish health and finding even one parasite warrants immediate treatment. Chilodonella is a potentially dangerous  parasite for two reasons. First, unlike many parasites, it has a wide range of temperature tolerance and outbreaks often occur at low temperatures when fish are least able to resist. Secondly, despite is relative small size it, is potentially more dangerous than Ich, because in the initial stages there are no readily visible signs of its presence.


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Chilodonella: Fish parasite

Chilodonella at 400x magnification (phase contrast) 

Chilodonella a dangerous parasite

Chilodonella as seen in a wet mount of a skin scrape

Photos: Frank Prince-Iles

Often by the time it becomes obvious that something is wrong there may already be severe tissue damage. As with other parasites there is also the risk of secondary infections resulting from lesions caused by the parasite's feeding activity.


The clinical signs are the same as all other parasite infestations. Heavy and laboured  'breathing' (judged by watching operculum movements), flashing and rubbing, skin cloudiness caused by excess mucus,  loss of appetite and lethargy. At a later advanced stage (which may be too late for treatment) fish often isolate themselves,  sometimes near the water surface or water return. They can also exhibit extreme lethargy with long spells laying on the bottom with clamped fins.

It can be easily recognized in skin scrapes and gill biopsies from its characteristic slow gliding movements, often turning in circles.

It has a flattened shape, heart-shaped with a notched end. There are bands of cilia on the ventral (bottom) surface, just faintly visible in the photomicrograph. They measure around 30 - 80 µm in length x 20 -60 µm width. - slightly smaller than skin-dwelling Trichodina.


Chilodonella is usually easy to treat with any proprietary parasite treatment, malachite and formalin, potassium permanganate, copper or salt baths.

"Koi Health & Disease" by Dr. Erik Johnson DVM is 200+ pages from Fish Health Practice - Readable by Beginners and delivered overnight from

Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.


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What if you could just get "Here's what to treat with." and no advice or education?

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Expanded Content by Dr. Erik Johnson, and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved