I think the dark is like the light for these fishies. Like maybe they are afraid of the light and when they go to bed at morning they have their dad's turn off the darkness so they could sleep.

Octodad Dadliest Catch is an independent adventure video game developed and published by Young Horses. It is a sequel to the 2010 freeware game Octodad. The game consists of controlling the protagonist Octodd in completing chores typical of the mundane suburban father. See also: Octodad, Creeper (Minecraft), Bugsnax, Indie Pogo, Irrlicht Engine.