Defenitely not an Echinodorus.
Your plant is a Cryptocoryne - most likely a Cryptocoryne wendtii of some kind. There are quite many variations of Cryptocoryne wendtii in trade. They are generally very easy plants to grow, adapting well to very different aquatic conditions. By sudden changes in environment, the leaves may fall apart (=melt) very sudden - but wait patiently a few weeks, and new leaves will start to grow from basis.
- and welcome to the wonderfull world of aquascaping.....