It reminds me a little of Terrence Malick's work in terms of its artful (rarely distractingly so) photography and tendency to cut away from the human drama to show (a little of) what's going on in the natural world beyond it. Otherwise it mostly reminds me of Kes. Both films are about English 15-year-olds who seem trapped in working class worlds without hope. Or with little hope. In Kes, Billy Casper finds a kind of freedom through a kestrel that he finds and tames. What hope there is for him lies in the possibility that he might be able to experience something similar when he grows up. In Fish Tank, Mia Williams finds a horse that she plans to free, but her own freedom looks as though it will only come, if at all, by way of an escape to some other place.
Although Kes is basically a story of despair it is loved by many people (mostly men from the north of England) because it strikes them as an unusually realistic and sympathetic portrayal of their childhoods. I can't see anyone loving Fish Tank in the same way, but that might be because it's about a girl in the south of England growing up in the 21st century. It's a different world from the one I grew up in (so is Kes, but less dramatically so). Non-English people sometimes struggle with the accents in Kes. I think that will less of a problem with Fish Tank, but there is still a danger with cockney accents: