Worms in aquarium

[Ama]

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?

 

[Colin_T]

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.