Worms in aquarium

I've planted an unknown grass into my old 1 gallon aquarium for an experiment
It was used as a hospital tank before
Then yesterday i found some white tiny worms in it... Like a lot of. Currently there isn't any fish in that Aquarium
But my concern is,Is that harmful for fish?
And what kind of worm is that?

The skinny round one looks like a detritus worm and is nothing to worry about. They normally live in the substrate but if the gravel gets dirty or the water goes bad, they make a break for it and try to get out of the water.

The flatworm thing could be a leech but it's a bit hard to tell from the picture. If there's no fish in the tank I wouldn't worry about it. Just do a big water change and gravel clean the substrate and they should all go back into the gravel.