The other idea I had on my next pond was instead of a dirt dam....and acrylic dam. A 4' x 15' piece of acrylic (same material used in zoo aquariums). The problem would be holding that seal. And the piece of acrylic would have to be oversized and buried...like how a anti seap collar works on a drain pipe. The seal would be the toughest part.
Then you got a nice viewing port....but your outside..not in a tube.
