EVEN as a kid taking a dip in a friend's swimming pool, Derek Spielman would think about how it would be much better if it was full of turtles. Now his own backyard swimming pool is one of 50 on Sydney's north shore that have been turned into ponds, saving thousands of dollars on power and water bills.

There are no turtles, but Dr Spielman's pool at his home in Gordon is swarming with native fish, such as empire and spotted gudgeons and Pacific blue eye. It's noisy with a chorus of frogs and full of water lilies and other aquatic plants, attracting native birds, dragonflies and bees.

Water relief … Callan Spielman wades in the family's former swimming pool, which has become a pond. It is now swarming with native fish and frogs. Credit:Mick Tsikas

There's not a mosquito in sight (or sound) because the gudgeons eat the mosquito wrigglers. And the mosquito species that live in water deeper than 30 centimetres don't bite humans.

''I wanted to turn our pool into a pond for years,'' said Dr Spielman, adding that it took some time to convince his wife.