Worm or leech in fish tank?

I found something in my goldfish tank. It probably came in with some live plants. It looks like a worm or a leech. It is brown with a sucker at only one end. It attached itself to the glass with the sucker. It is round not flat. It can scrunch up to 1/2 inch or stretch out to nearly two inches long. Do you know what it is? Are they dangerious to my fish? How do I get rid of them?