Make sure you did not miss the last part of the video, where the light rays were made visible. They probably offer better explanation than the text below.
This trick works better when the eye is set low.
Without the water: light from the fish undergo one air-glass-air refraction at the bottom wall, and a second air-glass-air refraction at the side wall, and arrives at the eye.
With the water: light from the fish undergo the air-glass-water refraction at the bottom wall. Since the refractive index of glass and water are much closer (than glass and air), the ray does not quite “unbend” itself. This causes the ray to attempt the water-glass-air refraction at too large an angle. At the glass-air boundary, the ray undergoes total internal reflection, and reflects back into the tube.
- The total internal reflected rays can be viewed when we peep into the tube from above. It appears as the inverted fish behind the side wall.
- With the eye placed low (and water in the tub), what we see at the “bottom” of the glass tube (the greyish metallic surface) is actually not the bottom of the glass tube. They are probably light that have undergone total internal reflection on the bottom of the glass.