How long can a goldfish remember things?

Answer

Experiments show goldfish can learn feeding times and be trained to respond to cues, and remember them for months.

Context

The three-second claim has no good experimental basis. Discover Wildlife reports on work at Plymouth University in which goldfish learned to push a lever to release food. When the lever worked for only one hour a day, the fish learned to nudge it at the right times, a skill called time-place learning because it ties a reward to when and where it appears. The same article states that goldfish can retain memories for at least three months.

Laboratory studies of fish more broadly show learning of colours, shapes and routes, and fish can be trained to associate a signal with food and respond to it later. These results come from controlled training and testing, in which a fish is rewarded for a response and later tested for it after a delay, the standard way memory is measured in animals.

Where the three-second myth came from is unclear, and I found no study that measured such a limit.

There is a difference between memory and attention. Short-term attention in any animal depends on the task, and showing months-long retention of a trained response does not mean a goldfish understands its tank the way a person would. Claims of owner recognition are common in pet writing but are less well documented.