1. [1930s+] (US Und., also fish tank) a holding cell in a police station [play on SE, the visibility of the prisoners].
2. [1940s+] (US prison, also fish tank) the processing unit for new arrivals at a prison [fish n.1 (6c)].
3. [1960s] (US) steam baths frequented by homosexual men.
4. [1970s] (S.Afr.) a sleek car, e.g. a Cadillac.