You mean a tank that has a tank connected so fish can swim between? If so try an upside down fish tank. I think there's alot of plans on the internet for you to browse
Not so much the cascading although those are pretty sweet looking. More so multiple tanks so fish can swim from one to the other. I have searched many websites just can't find a plan that seems structurally safe. I'm real picky when it comes to the construction aspect.
hmmm, we have what your chasing at work kind of, its a 3ft tank with a 5-6 inch hole connecting it to another 3ft tank. theres a poster saying only 5 in vic (where I live) so not sure how easy they are to find.
Personally I would build a cubed tank but leave the middle open. then you can have your filter in the middle or a waterfall if you wanted.
I think if you want tunnels etc will need to be acrylic.