Proserpinaca palustris
Mermaids Weed
This plant is sometimes called mermaids weed. The first time I got it, it was under the name Australian Hygro, but it isn't from Australia nor is it a hygro. This is a US native plant. It's very interesting and attractive but exremely slow growing without CO2. There are 2 types in the hobby. The new one is called "Cuba" and is the variety in the photo.