Hundreds of fish died and others were left fighting for life after pollution hit the Clock House gravel pits in Bedfont this week.
Environment Agency officers were called to the gravel pits on Tuesday after dead fish were spotted floating on the surface of the water.
They managed to clear the casualties, which included perch and small fry, and rescued the remaining distressed fish.
Oxygen was pumped back into the water by aerators.
Environment officer Geraint Rowland said: "This is a serious incident, which has affected a stretch of water with both sporting and natural value. We believe we have now identified the source of the pollution and that it is no longer getting into the water."
Although the cause of the pollution has not been revealed, it has been suggested it could have come from Heathrow airport.
BAA is involved in the investigation.
Managing director of Princes Ski Centre, which uses the gravel pits, Glen Walker said: "We are working closely with the Environment Agency and BAA to monitor the incident but for now it's business as usual for us."
Nearby Bedfont Lakes Country Park was also alerted to check the wellbeing of their fish.
Ranger Chris Pate said: "There was worry the water from the gravel pits ran into ours but this is not the case. Our fish are fine."
Members of the public can report pollution incidents by calling the Environment Agency's emergency hotline on 0800 807060.
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