Video: How could anyone go two years without noticing something this huge? Well, humans have been around for 200,000 years. Bristle worms and their polychaete brothers have survived five mass extinctions. Many aquarists fill their tanks with live rock, that is, chunks of coral that still harbour biological and bacterial matter. Bristle worms often piggyback on these corals, infiltrating a tank and growing to tremendous size. Generally they remain hidden beneath rocks and coral, poking out at night to feed.
Some people choose not to get rid of them. On the one hand, they're scavengers who clean detritus and generally don't bother fish (although they can eat coral if they're desperate). On the other hand, their tiny spines make them painful to handle -- another aquarist described the feeling as analogous to fibreglass -- and therefore difficult to remove. In this case, the worm grew to almost 1.2m long and draining the entire tank was the only way to get rid of it.