Why does the fish only remember for seven seconds?

The original joke of "fish's memory is only seven seconds" is:

As early as a long time ago, someone told me that a fish's memory is only 7 seconds. After 7 seconds, it won't remember what happened, and everything will be a new beginning. So, in that small fish tank, it will never get bored. So many people mistakenly think that the memory of fish is only seven seconds.

In fact, fish's memory is not limited to 7 seconds. This is just a literary statement.

Extended data:

Scientists' research overturns this view. They found that fish not only have memories as long as five months, but also have a certain "concept of time". The British "Daily Mail" reported on the 6th that three scholars from Israel Institute of Technology, Boaz Zion, Asaf Barchi and Ilan capra, conducted an experiment:

Play a certain sound with the speaker every time you feed the fish. After a period of training, as long as you hear this sound, the fish will come back to eat. ? 1 month later, scientists put the fish into natural waters and let them swim at will. ? After four to five months, the three people played the original training voice again, and the fish followed the voice again. ?

Scientists say that this experiment is based on the principle of conditioned reflex. The artificially raised fish will have conditioned reflex after being repeatedly stimulated by external sounds. A few months later, when fish hear this sound again in the wild, they can make the same reaction because they remember it.