Anyidea why laser pointers used on a fish tank make the fish freak out?

They freak out because the laser is bright and the water dissipates the light throughout the tank.

Lots of people with aquariums use laser pointers to play with the fish. The reaction is typically that the fish chase the point of light. Hard to say if they’re trying to snack, or if it’s stimulating their territorial response. Most fish are intensely curious of anything new in their environment, kind of like cats are. No surprise then that both cats and fish love the lasers.

Here’s a video.

I’d probably freak out if a giant red beam of light suddenly appeared in my world!!!