I've seen these long creepy guys crawling in my tank, but they never harmed my fish, while I never thought if they are good or bad... I love these guys, I don't care if they're poisonous or sting humans, they clean the food hyper fast, better than any clean up crew animal, some of them are red, maybe fireworms and I once tried to get one out with tongs but they're so strong that the worm cut himself in half