Fish Lice are one of the few parasites that are not microscopic. In the mature form, they appear on the fish’s surface, 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 a koi’s slime coat. 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.