How long have you had it?
I wouldn't worry about it. It doesn't seem to have lost leaves and the ones it has look healthy aside from hanging down a little.
It's looks to me like the lower portions aren't getting enough light. See how the spacing changes so abruptly going up the stem? I'm betting that the amazon sword is overshadowing much of the plant. You should move them to a different, more open location.
See how all the leaves have fallen off at the very bottom? That also makes me think of a light issue.
Good luck.
Maybe. How deep is the tank though?
I'm doubtful though. The plant looks great at the top and just goes in the crapper fairly rapidly. It might just be a total light issue. Is it getting shaded there like I think it is?