What is a goldfish thinking?

The three-second memory is a myth, and a well-tested one. Goldfish have been trained to push levers at set times of day, to navigate mazes, and to still remember the routine months later.

What they are genuinely poor at is remembering a featureless tank, because there is nothing in it to form a memory of. That is where the reputation came from, and an enriched tank changes the behaviour noticeably.