Its voracious appetite has cleared polluted water and allowed plants and birds to return to Ormesby Broad near Great Yarmouth. The project, the largest of its kind in Britain, is being watched closely in Europe. Ormesby, covering 54 hectares, is an important part of the Broads network. But water quality has declined due to a growth in algae.
Biologists have removed 9.5 tonnes of fish which prey on the fleas, who devour the algae. The fleas thrived and the water is now clean again, an event marked by an open day organised by the Broads Authority.
A glass-bottomed boat gave free rides to show how underwater conditions had improved. Plants, like stonewort and horned pondweed, have germinated from dormant seeds.
Birds including coots and swans have also returned. Conservationists will monitor the lake's progress and eventually recolonise it with coarse fish.
Jane Madgwick, chief conservation officer for the authority, said: "If we can do that it will be a real breakthrough. This project is being watched carefully abroad but nobody has taken a scheme like this through to completion."
She said it would take about five years to get the whole ecosystem back in balance. But the first part of the process had succeeded. The scheme is funded by the European Commission, the Broads Authority and the National Rivers Authority, who are co-operating with the Suffolk and Essex Water Company, which owns a large part of the broad.