the current walk-through (a curved acrylic tunnel) is a 2014 refit, replacing the original straight-walled tunnel which has been there for 30-odd years. The current tunnel was put in by a company called Aquarium Technology Limited. On the following link you can see a photo of how the original tunnel looked, as well as the new version.
Bristol Zoological gardens Aquarium New walkthrough acrylic tunnel
Bristol Zoological gardens Aquarium New walkthrough acrylic tunnel