Not a good thing. A very bad thing. A thing that you want out of the tank. A thing to be avoided at all cost.
A thing that is "an aquatic predatory polychaete worm dwelling at the ocean floor at depths of approximately 10–40 m.
This organism buries its long body into an ocean bed composed of gravel, mud or corals, where it waits patiently for outside stimulus to reach one of its five antennae.
Armed with sharp claws, it is known to attack with such speeds that its prey is sometimes sliced in half. Although the worm hunts for food, it is omnivorous.
Little is known about the sexual habits and life span of this worm, but researchers hypothesize that sexual reproduction occurs at an early stage, maybe even when the worm is about 100 mm in length; this is very early, considering that these worms can grow to sizes of nearly 3 m in some cases (although most observations point to a much lower average of 1 meter) and 25 mm in diameter. A long lifespan may very well explain the size of these creatures."