Paddy Ashdown, the Liberal Democrat leader, revealed the heatwave drama when quizzed over the joke shared by the three main party leaders when they were filmed smiling together during a break in the VJ Day commemorations.
During the 10-minute aside, "the Prime Minister gave us a very engaging description about his goldfish. It had sunstroke over the holidays and had to be taken out and given some sun cream," Mr Ashdown told a Westminster news conference.
The fish, said to have survived the ordeal, lived with a number of others in a pond in the Prime Minister's Huntingdon home. Number 10 routinely declined to give details of the first aid exercise, which was "private".Tony Blair's office was similarly tight-lipped: "It's not our goldfish to talk about."
It was one of the worst summers on record for UK fish - although experts doubted they suffered sunstroke. Ray Butcher, a former president of the British Animal Veterinary Association, said vets would treat sunburned fish with a medicated cream, although in some cases, a normal sunblock might be used.