Fish tank
luca medium.com
I agree. The entire film is centred on Jarvis’ performance. Speculations of her potentially concomitant life to that of Mia’s aside, she is perfect for the role. This film would not be the same if it…
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Medium
The esteemed critic Roger Ebert, of the star of Fish Tank Katie Jarvis, says;

In a film so tightly focused, all depends on Katie Jarvis’ performance. There is truth in it. She lives on an Essex housing estate like the one in the movie, and she was discovered by Arnold while in a shouting match with her boyfriend at the Tilbury train station, which is seen in the movie. Now 18, she gave birth to a daughter conceived when she was 16.

We can fear, but we can’t say, that she was heading for a life similar to the one Mia seems doomed to experience.

I agree. The entire film is centred on Jarvis’ performance. Speculations of her potentially concomitant life to that of Mia’s aside, she is perfect for the role. This film would not be the same if it was Margot Robbie, Angelina Jolie, or Jennifer Aniston in the lead role. Jarvis brings a rawness, a purity (ironic) and a grit which is inseparable from and entirely representative of this poverty-stricken English locality.

There is goodness in this family’s heart. As Mia sets off for Wales, we may never know if it is realizable through conversation, therapy, and rehabilitation, or if it can only be unlocked by dancing to early 2000s / 90’s CD’s. We are left wondering what is next for Kiera – presumably, she is destined for the same teenage angst as Mia, for the same neglect, abuse, and corruption. The only way through is out. Fish Tank presents exactly that; a fish tank, an all-consuming, claustrophobic ecosystem of violence, anger, hatred, sexual perversion, neglect, and substance abuse. Perhaps Kiera too will have her moral epiphany, see the death of her own white horse, and leave this moral wasteland. If she doesn’t, she is destined for the inescapable corruption which makes this fish tank of poverty just so effective.

Presumably Mia makes something of herself in Wales, or perhaps she doesn’t. We won’t know. We may guess that her years of abuse and neglect have rendered her unsalvageable – that she is destined now for a life of corruption due to her childhood trauma, even if she has left the fish tank. If this is the case, we must worry for Kiera also. In many ways, the tragedy of the film is not the journey of Mia in and of itself but the precedent it provides for the life of Kiera, who cannot yet slink off to another country (Wales counts, technically) and must endure.