The mirror effect is for the wall of the aquarium, to let the main character beluga see its shadow, which gives more sense of isolation. I downloaded the glass material asset, changed the parameter and added scratched texture which can simulate the horror effect.
To make the reflection works. the key is to set some parameters for lumen and ray tracing in the project setting.
I cut half of the sewer mesh asset and assign glass martial on it. And here is what I got: