little black worms in fliter

Question
i have these smooth little black worms in my filter i had gold fish when i found them dead those worms were in the filter didn't have fish very long now i have molly's and went to clean the tank and found black worms again what are they and how to get rid of them are they harmless?

Answer
Aubrey,

Have you ever fed live food such as blood worms to any fish in that tank? What are the water perimeters? How large is the tank and filter. When was the last time you cleaned the filter and how often? Did you clean the entire tank after you found the dead goldfish? Are they black or red? As soon as you get me the answers to the question I can help you out better.