It’s terrific to see a gifted director coming into her own, and a young, talented actress grabbing a role and refusing to let go. However, Fish Tank winds up being a major downer, because the director and the star are doing an effective job of depicting miserable, messed-up lives. Newcomer Katie Jarvis is outstanding as Mia, a 15-year-old girl who is pissed off, for many reasons. When her mother brings home a new boyfriend, her already troubled home life is further complicated. Michael Fassbender, as the mother’s boyfriend, is the worst kind of movie monster, a person who appears admirable but is nothing but human garbage. Jarvis and Fassbender deliver two of the year’s best performances so far, and director Andrea Arnold delivers her best work yet.