The Anacharis is a great beginner's plant for your fish tank. They're also known as the Brazilian waterweed and as you can guess, since it's a 'weed' it can grow quite fast and easily in your tank. It has bright green leaves that are covered in bunches of smaller leaves. It roots down into the substrate but can also grow floating at the top of your fish tank.
It grows well in either tropical or cold water tanks. In tropical tanks, it does require regular additions of fertilizer to help it grow faster. When floating at the top, they do well since they are closer to the light source and they provide a safe shelter for fishes like livebearers.
Keep them in a tank that ranges from 59 to 82F. You can trim them by cutting them and either tossing them or propagating them by giving them away. Because of the fast growth and size, they do well in the back of your tank as a background.