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Like dogs, cats & parrots goldfish have very sharp memory. They can even remember things for a few months also. There is a myth : 3 second goldfish memory. Actually it's a bogus. Scientist have already proved that goldfish have a very long memory. Here is the experiment researchers of Technion Institute of Technology in Israel did -

First they trained baby goldfish to associate a sound played through a loudspeaker with feeding time. Every time when researchers played a particular sound, the baby goldfish would return for food.

After one month of training, researchers released the baby goldfish into the sea from a cage. Five months later they had become adults. Then scientists played that specific sound and all adult goldfish returned to their cage.

References:

http://www.dailymail.co.uk/sciencetech/article-1106884/Three-second-memory-myth-Fish-remember-months.html#ixzz4kL2fnfNa

http://todayilearned.co.uk/2013/07/05/goldfish-actually-have-relatively-long-memory-and-can-remember-things-for-months/