Skinny Worm ID?

Somehow, in a bucket of aged tapwater I was about to add to my aquarium
I found I strange, very skinny worm. It's about the general size of a
bristle worms, and it does, indeed, have bristles, but they are smaller
and harder to see than those on other bristleworms I've seen. Also, it
has two "feelers" at its anterior end, and is dark brown rather than
orangish. Is it harmful or beneficial? I can't imagine how it got in my
water bucket as I always use different buckets to remove old tank water
than those I use to add water. But my inclination is to throw it back
in my tank (120 gallons). Is this enough of a description for an ID?
Thanks.

Yours truly,

John Mayer