Fish Lice are one of the few parasites that are not microscopic in size. In the mature form, they appear on the surface of the fish, usually as a small dark green or brown spot that will move throughout the day. This parasite will attack even a healthy koi and is not affected by the natural parasite inhibiter in the slime coat of a koi. They are not a real threat on their own but often cause secondary bacterial infections. Fish lice have a tiny stiletto like mouth that they use to puncture the fish and draw out blood.