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 .