Arnold's central character, 15-year-old Mia (Katie Jarvis), is a flawed but feisty heroine, living in a public housing complex on the scruffy fringes of London.
Expelled from school, at loose ends and ostracized by her former best friend, Mia shares an apartment with her mother and younger sister
- and, as the film starts, with her mother's new boyfriend Connor (Irish actor Michael Fassbender), whose arrival destabilizes the already shaky family.
Arnold said she hopes "Fish Tank," playing at the Cannes Film Festival, "shows one of those teenagers in a more rounded way."
It succeeds in large measure because of Jarvis, who had never acted before and was spotted by the filmmakers arguing with her boyfriend at an English railway station. Jarvis, now 17, wasn't at Cannes for the film's premiere
- she had a baby two weeks ago.
Her performance as Mia is raw and real, both aggressive - at one point Mia headbutts another girl
- and vulnerable.
Arnold, 48, said Jarvis's inexperience as an actor became an asset.
"Sometimes knowing too much can be a problem," she said. "She didn't know very much, so had no fear in a way."
The performances feel natural, but "Fish Tank" is also full of artful images that capture the beauty in an area few consider attractive
- London's urban fringe of apartment towers, highways, big-box warehouses and the wind-swept estuary of the River Thames.
"I don't know why people don't see it as beautiful," Arnold said. "It's got really fantastic wild spaces. Even that block of flats
- there's lots of people there, loads of kids, lots of energy. I don't see it as a bad place."
Arnold won an Academy Award in 2005 for her short "Wasp" and her first feature, "Red Road," won a prize at Cannes in 2006. "Fish Tank" is one of 20 films competing for the festival's top prize, the Palme d'Or, which will be awarded May 24.
It's Arnold's biggest-budget film to date, but cost only 2 million pounds ($3 million) and was shot in 30 days
- not an experience the director wants to repeat.
"I was hearing that Terrence Malick when he made 'Badlands' would spend three or four months just hanging out," Arnold said. "And I thought,
'Yeah. I fancy that.'"