I have a small pond in the backyard that houses 10 goldfish. When I was cleaning the filter last, I saw some red colored worms in the filter pad. They look like blood worms, but I'm not sure exactly what they are. They are about a quarter of an inch long and blood red, and there are dozens of them grouped on the pad. I would go and take a picture but its the middle of the night. Maybe I'll get one taken in the morning, when the sun comes up. I would rinse them thoroughly under tap water before introducing them to my betta.
EDIT: I just did a quick bit of research and found them to most likely be midge larvae, or similar larvae from the Chironomus species, but could also be actual tiny worms from the Polychaeta family.