Why is the movie called 'Fish Tank'?

I got an impression that it was perhaps a metaphor for Mia's life?

A Fish tank, a blank and endless circle, where like a fish, she would circle for ever in a sort of unfulfilling haze. Hence, the reason her leaving with the 'Pikey' boy was so important, she 'escaped' the fish bowl.

'You see, I'm not a Monster. I'm just ahead of the Curve'

I read it was because Mia's world is so small. Little apartment, very few friends if any, pretty much alone. Just like a that of a fish tank.

Agreed. I think "Fish Tank" is metaphorical for Mia's life. I also saw the fish that Connor catches as representing Mia. He catches the fish very gently and then the next morning it's gutted on the kitchen floor, just food for the dog. And when she's working out her audition routine, the sound is gone and replaced with the sound of being underwater and she's moving like a fish in a tank - back and forth.

As obvious as it might be, I just saw it as the room where she went to practice her dancing.

personally i took it as one of two ways, first that it is called fish tank because mia like a fish is stuck inside her tank never unable to escape her situation, and two i thought it might refer to the fact that she is constantly followed by the camera and so we are constantly viewing her as our form of entertainment, much like a fish in a tank she is constantly observed by the viewer and can never escape it we are effectively watching her (the fish) swim around aimlessly in her council estate (the tank)

I thought "fish tank" was a metaphor for how our environment affects us. Mia is a troublesome teenager, her mom acts like she was born that way. Remember when she was talking to that social worker and said "She used to cry for days when she was a baby, she was born looking for trouble"? Yup, her mother is a horrible woman who doesn't take any responsibility and that's why her daughters are the way the are, but obviously she blames THEM for being the way they are.

In a scene in the house we see the fish already dead in the kitchen after they caught him, it was being eaten by the dog, they didn't even get the chance to cook it. Even though Connor gave the fish a merciful death, it died a useless death (it didn't become a deliciously cooked fish; it became garbage and dog food instead). That fish could have had a longer life in his natural environment... or even in a tank. I saw this as the scene where the author tries to show us the main intention of the film: to depict the reality of people who grow up in nasty environments and end up badly because they never had the support they needed, and their only option for their future is a painful existence and a meaningless death.

What is never mentioned is if Mia got pregant or not after she had sex with Connor (I was expecting him to mention the possibility of taking the morning after pill when they had sex, but if she didn't, she probably will get pregnant in Cardiff, or at some point in her teenage years. She will grow up to be her mother and end up living a miserable life... because she was born in the wrong tank.

Life... Life is a pigsty...

After reading many of your insightful responses it came to me the title, Fish Tank, eludes to a way of life. Tenements where single mothers raise their children. No man in residence. Mothers so young and horny, careless about protecting of their young daughters. All mom wants is a young stud. Oblivious to the needs of their daughters starved for attention, especially male attention.

The fishing scene is a metaphor for who Connor is: a pedophile. He doesn't need a fishing pole, line or hook. He just uses his wiles, charm, good looks and skill to bring the fish to him. First Connor shows interest in Mia's dancing. He gives the girls treats: taking them for a car ride into the country. He plays games with them like giving the little sister money for letting him out the front door. He brings Mia a camera to record her audition, but he's her first subject . . . dressing and undressing. He feeds into her voyeurism.

After catching the fish he skewers it with a stick, which foreshadows sexual penetration.

Next scene Mia is bleeding, which symbolizes lost virginity.

The next day the fish lay dead on the kitchen floor, the dog eating its flesh. Connor didn't care about the fish as food, he just enjoys the hunt.

Wow, there's some good explanations here. I didn't even think to link it to that fish Connor caught.

I thought it was called Fish Tank in reference to the council estate. Mia's often walking through the hallways and you can see and hear other people's lives easily -- their arguments, their music, their kids running around. Mia's mother's apartment door seems to be always open. That sort of life is very transparent to others and there's not a lot of privacy. Mia has to make do with unconventional spaces (like that empty apartment where she dances) when she needs privacy. Otherwise people from the outside -- like Connor, or that social worker -- are eager to take a glance into council estate life, but they leave right away. There's no advantages to living in a fish tank -- it's transparent and cramped. Basically a trap for those who need to get away.