There is a popular belief that goldfish only have a three-second memory span and every lap of their fishbowl is like seeing the world for the first time.

Children's films like Disney's Finding Nemo - in which one of the main characters is a dory who suffers from short-term memory loss - have done little to help dispel the myth that fish are dumb.

But a 15-year-old schoolboy from Adelaide has just debunked that theory.

He has conducted a simple experiment, which proves that the humble goldfish is smarter than we think.

"I mean it never really seemed feasible to me, that they had a three, five, 10 second memory because animals need their memory, so they build up over time a knowledge of where the food is," said Rory Stokes, a student at the Australian Science and Mathematics School.

"It seemed pretty impractical for a species to evolve without these capabilities."

He conducted a school experiment to prove his theory, using a small tank of goldfish.

"I decided to get a bit of red Lego and just feed them next to that. Every day I'd put it in and sprinkle food around it," he said.

"At first they were a bit scared of it, a bit wary, but by the end of the three weeks, they were actually almost coming before I put the food in."

After leaving the fish alone for a week, Rory placed the red Lego block in the tank again.

"They remembered perfectly well," he said.

"They actually had a time faster than the average of the three feeds before I left."

The goldfish showed that not only could they retain information, they also had the ability to recall it at a later date.

Culum Brown, a research fellow at Sydney's Macquarie University, has studied fish behaviour for more than a decade.

He says his studies of Australian native species prove fish are intelligent creatures that know how to avoid predators and catch food like any other animal.

"The thing that I really liked about Rory's experiment is he not only got that classical conditioning going [but] the fact that he could get them respond just to that specific coloured marker I thought was really good," he said.

"One of the experiments I did was looking at teaching fish to escape an artificial trawl.

"The net had a small escape route in it and the fish just had to learn where that hole was and they learned that in about five trials.

"If you put the fish aside and test them a year later, they still remember exactly where the escape route is."

It is not a bad feat for an animal with a brain 380,000 times smaller than a newborn human baby.

But if fish are smart, it raises the question of whether it is cruel to keep them in tiny fish tanks.

"Definitely - I think the more complicated you make your fish tank, the happier your fish is gonna be," Dr Brown said.

"In fact, the best thing to do is to keep changing it around, so make it interesting for them.

"You'll find that if you do that, your goldfish will be much more active and much happier in general."