| | Re: Echindorus 'xingu
Yes, it falls under Helanthium, and is something from the Helanthium bolivianum group. There's a big mess in the trade with different Helanthium forms and their names. It may be that different plants are called "Echinodorus xinguensis". Ones being different from E. latifolius as well as the same stuff. The same problem with other names for broad-leaved Helanthiums in the trade such as E. magdalenensis, E. quadricostatus etc.
I don't know if there are cultivated plants that are identical with the plant scientifically described as E. xinguensis.
Perhaps Daniel Falck knows more about the issue.