This is a ciliated protozoan that attacks fish including discus fish. It is called Cryptobia. This is an internal disease of the fish where the Cryptobia organism is creating granulomas in the fish.
Symptoms of Black Stripe
Most times the coloration is always black and it is seen on the head alongside one or two vertical stripes on the body as well. And some other times only the head or the tail turns black.
Causes Of Black Stripe
The cause of this black stripe in fishes is not clear, but some references on the internet affirm that the stripe does not have pin pointed cause.
Discus Black Stripe Treatment
There is no medication or treatment for it. It often just goes away on its own and doesn’t seem to be very contagious.
Research shows that this protozoan cause black pigmented areas to appear on Discus fish, and these are most times in large stripe, while some other parts of the body and fins usually will take on a lighter and faded out coloration. These stripes are called the black-stripe syndrome.