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9 April 2012 Last updated at 10:57 BST
The children have been breeding rare 'brown trout hatchlings', and it's taken them three months.
They're doing it to get hundreds more trout back in the Wandle, a river in London.
In the past, all the trout there had died because of pollution.
Watch Joe's report as he joins them on their mission.
Children from seven schools in London have been rearing their own endangered fish as part of a school project.