They are the smallest and toughest survivors of the earthquake that struck Christchurch last February.

Two goldfish spent 134 days in a tank with no one to feed them or clean their water. Discovered earlier this month when their owner, Vicky Thornley, returned to her office in the ravaged city centre for the first time, Shaggy and Daphne – named after characters in the cartoon Scooby Doo – are believed to have survived mostly on weeds and algae. They may have had another source of nutrition, for three of their companions were missing from the tank at Quantum Chartered Accountants.

Ms Thornley, the company director, was about to step into a lift when the 6.3-magnitude earthquake hit. She grabbed the tank, both to stop it from falling and to steady herself. "I was clinging on for dear life," she said.

Ms Thornley and her three colleagues managed to escape from the building, located in Christchurch High Street, but her first thoughts were for her five-year-old son, Joshua, who turned out to be unharmed.

With much of the city centre destroyed, it was not until three weeks ago that she was allowed back into the office – and only for an hour, under escort. She didn't look in the tank, assuming the fish were dead. But she said a recovery worker yelled to her: "Hey, there are fish here and they're alive!" Ms Thornley said: "I was astounded."

The goldfish looked dull in colour, but otherwise appeared in good health. She scooped them out and took them home in a bucket to her son Joshua, who has two fish of his own, Scooby and Fred.

According to Paul Clarkson, curator at California's Monterey Bay Aquarium, the survivors were fortunate to live in a large, 100l tank with plenty of weeds to eat and algae growing on the rocks and walls. While there was no electricity to power their tank filter, naturally growing bacteria may have helped keep their water clean enough to sustain life.

Goldfish: the facts

* The biggest goldfish ever caught in the UK measured 18-inches long and weighed 7lb 5oz. It was found in a lake in Norwich.

* If a goldfish is kept in the dark it will eventually turn white.

* If kept in a tank, one gallon of water is recommended for every inch of goldfish.

* Some types can live for up to 20 years.