I love these tanks and fish. A perfect reminder that small fish are every bit as interesting as larger ones; don't need to be "monsters".
I say this whenever I see a biotope-type tank that I like, but...I really, really need to try one of these set-ups. I'm toying with the idea of installing it above a larger "normal" tank, and re-routing the sump return to flow through the length of the rheophilic tank before draining into the main tank at the other end.
When you mention finding fry in other tanks, do you mean that the fry have managed to work their way through the overflow drains and into other parts of the system?