It looks like Cardamine lyrata to me. I had some in my 75g back when I was running it as a high-tech tank.
Bump: Here are pictures of it in my 75g. It's on the far right side of the tank, looks rather whispery with very thin stems. It never liked my tank that much, but it's supposed to be pretty easy to grow.
Pictures of the Cardamine lyrata n my 75g.