Little red worms in my aquarium

Hi all, complete novice here. I hope I'm not asking a question that has been answered several times.
A couple of weeks back I got a new tank to start a planted aquarium. Its half substrate/planted half sand.two days ago i noticed near to 200 little red worms poking out of the sand. Theres no fish in the tank atm. So I emptied the tank changed the substrate and boiled the sand before setting up again. Everything was given a thorough clean, but I'm still seeing some flapping around in the...

Little red worms in my aquarium