new plants

I have no clue as to where these plants came from. I am setting 
up a 10g. All it has in it is water rocks and an ugf. About a 
week ago plants started growing in it! They look like a blade 
of grass with 3 little flowers on the top. They grow at about a 
rate of 2 in a day!! I have no clue as to what they are or 
where they come from! Please help!