I have been looking to build one of these myself. The place I have seen what you may be talking about through carolina biological - their River Tank Ecosystem.
http://www.carolina.com/product/161536.do
I hope this is at least similar to what you have been looking for. Some of the other ones I have seen have places for lizards and frogs.
I'm still trying to figure out what I am going to do myself for a setup.
Jillirie - Would you happen to have references or websites which you used for guidance to build your setup?
Mike