July 2026

Quick Answer Koi flukes are microscopic flatworms — Gyrodactylus (skin fluke) and Dactylogyrus (gill fluke) — that are invisible to the naked eye, unlike anchor worm or fish lice. You can’t confirm them by looking; you confirm them by behavior (flashing, clamped fins, gasping at the surface, reduced appetite) and by a skin or gill […]

Quick Answer Anchor worm (Lernaea) looks like a thin, thread-like white or green filament sticking out of your koi’s skin, while fish lice (Argulus) look like flat, translucent brown discs about 4-10mm wide that move freely across the fish’s body. Both are crustacean parasites, not true worms or insects, and both thrive fastest in water