Why do the sides of my fish aquarium look like mirrors?

I bought a new fish aquarium which is square/cuboid in shape. My previous aquarium was spherical in shape. When I filled it with water and put the fish in two of them started acting very stressed so I was a bit worried. When I looked into the aquarium from the top, the sides of the tanks looked like mirrors under the water line. This caused me to worry that the sides of the tanks were mirrors to the fish and they could not see through the glass and into the room. Because of the stressed behaviour of the fish, I was almost convinced that this was the case and that the fish were in effect, in a prison made of mirrors. I tried to find a way of seeing into the sides of the tank like the fish so I put a mirror under the waterline and the mirror reflected the images of what was outside of the aquarium glass. So I thought that the mirror effect was an optical illusion but without putting my own head under the water and looking through the sides of the aquarium, I can't be 100% certain. Can anybody confirm if this is an optical illusion?

The glass-to-air interface will partially reflect too, but there will be total internal reflection for light rays incident at greater than the critical angle, which has a value of $$\sin^{-1} (n_2/n_1)$$ (for $$n_2 < n_1$$), where $$n_2 \simeq 1$$ for air and $$n_1\simeq 1.5$$ for glass.