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I have a garden pond that suffers from 鈥減ea-soup鈥� algae. A thunderstorm recently flooded the nearby road, overflowing into my garden and the pond. The next morning the algae had gone. What was in the floodwater that managed to clear the algae?

Colin Carter, Marlborough, Wiltshire, UK

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This article appeared in print under the headline 鈥淧ond skimmer鈥�