In running some nitrification studies, despite 50% water changes per day, the nitrite levels were well above 0.5 ppm (approaching 5 - 10 ppm) for over 2 weeks. We noticed 2 fungus like pinkish cottony tufts (about 2 -3 mm in diameter) on the top and bottom of just one butterfly koi's caudal fin. These tufts appeared to be a raised 3-dimensional spherelike growth that have been known to grow to a height of 2 mm..
|Epistylis on caudal fin|
|Epistylis on side of Koi|
|Epistylis and Trichodina|
Click to enlarge
The water had 0.3% salt from the beginning of the nitrification test. This level of salt was not enough to prevent the epistylis from forming with this high level of nitrite.
According to the literature Flexibacter columnaris is typically the primary infection. It takes nourishment from nitrous acid (from the nitrites) to reproduce. The secondary attack is by the ciliated protozoan epistylis. This can then develop into a 3rd attack by Aeromonas eventually resulting in the death of the Koi. Many believe that epistylis can lead to "hole in the side" disease.
The Epistylis is reported to be sensitive to higher concentrations of salt 3.0%, 25 ppm of formaldehyde, organophosphate compounds like Dylox (Masoten), and swabbing with tincture of iodine .