Why would a fish keep swimming directly into the corner of the tank if it has been swimming very quickly up ans down the corner for about three days?

THE GLASS AT THE CORNERS CREATE A MIRRORED EFFECT BECAUSE ITS BEVELED SO THEY MAY THINK THEY SEE ANOTHER ONE OF THERE KIND. FISH ONLY HAVE A 3 SECOND MEMORY SPAN.

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Answer:

Fish DO NOT have a 3 second memory span. There's no way they'd survive in the wild if they did. Any1 who's every owned fish and actually interacted with them can tell you they remember a lot longer than that. They are especially good at recognizing their owner and each other.

People just say that so they don't feel bad about abusing their fish.