Here's why:
* Fish don't rely on light to swim: They primarily use their lateral line system to sense water currents and vibrations, allowing them to navigate and avoid obstacles. This system works even in the dark.
* Sideways swimming can indicate stress: If a fish is swimming sideways, it might be sick, injured, or experiencing stress from environmental changes.
* Nighttime behavior: Some fish are nocturnal and may become more active at night. They may move differently in the dark, but this is not necessarily sideways.
So, the idea that fish swim sideways when the lights are turned off is likely based on a misunderstanding of their behavior and how they navigate.