Tiny Worms In Tank

I was cleaning my tank when I noticed that there were a lot of tiny worms in
my gravel. The funny thing is, I have never fed my fishes with worms before.
They have always been fed on pallets.

Any ideas on what the worms are? Are they harmful to fishes? Will adding
0.03% salt kill them? I strongly belief that my canister filter is also
infested with the worms (I am reluctant to throw away the filter medias as
they are cycled)..

Any help is appreciated. Thanks