many stem plants such as Ludwigia or Hygro. hornwort or vals grow crazy fast for me. water sprite..... there are more depending on your lighting set up co2 and ferts.
My first plant, anacharis... Easy to grow even with no ferts or co2, and is a nice green, as long as you can keep it nice and thick, it will look really nice. It even did well in my 20h when I oly had 15 watts of light over it.