Fry like to hide and feel secure (safe) and live bunch plants are as close as possible to their natural environment. If they were to be in the wild, they would never come out of the plants because it's part of their survival instincts. This is why I use water sprice.
I think water wisteria works in the same way.
Glad to hear they like it. When you get a chance I'm sure I'm not the only one who'd like to see them in some nice pictures