Green’s Dictionary of Slang

1. (US Und., also fish tank) a holding cell in a police station [play on SE, the visibility of the prisoners].

1939		Howsley Argot: Dict. of Und. Sl. 18: fish tank [...] room containing prisoners awaiting investigation or their turn to be called to court.
1942		R. Chandler High Window 213: ‘I’m going the way I always go,’ I said. ‘With an airy smile and a quick flip of the wrist. And with a deep and heartfelt hope that I won’t be seeing you in the fishbowl. Good night.’.
1949		R. Chandler Little Sister 68: Maybe I get tossed in the fish-bowl.
1971		T. Thackrey Thief 192: Because if he wasn’t, you’d be in the fishbowl right now.

2. (US prison, also fish tank) the processing unit for new arrivals at a prison [fish n.¹ (6c)].

1949		Monteleone Criminal Sl. (rev. edn) 84: fish tank The main compartment of a jail where prisoners are first received.
1954		(con. 1941) C. Chessman Cell 2455 217: We had our heads shaved [...] and then assigned single cells in what was then the ‘Fish tank’ section of the Old Prison.
1982		Rapid City Jrnl (SD) 17 Jan. 31/1: An inmate and his kid discussed escaping. Their plan [...] to pose as fish during flag rec, enter the fish tank and then climb out of a vent.
2002		(con. 1998–2000) J. Lerner You Got Nothing Coming 21: I have already figured out from listening to the dawgs on the bench that the Fish Tank is the convict name for the intake processing unit at the prison. Like a boot camp barracks before being sent overseas or to a permanent station.

3. (US) steam baths frequented by homosexual men.

1969

C. Himes Blind Man with a Pistol (1971) 155: Next to it [...] was a steam bath establishment calling itself the Arabian Nights Baths. ‘That a fish bowl?’.

4. (S.Afr.) a sleek car, e.g. a Cadillac.

1977

J. Waring Hot Air 26: We step into our Cadillacs [...] and sweep off in our black fishbowls. [...] The Cadillacs, enormous cars, attract too much attention. So when you draw up at a robot [...] people peer into the ‘fishbowl’.

5. (N.Z. prison) an observation cell for unstable or ‘at-risk’ prisoners.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 69/1: 2 an observation cell for mentally unstable and/or 'at-risk' inmates.

6. (N.Z. prison, also fish box) the control room or guard house.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 69/1: fish bowl (also fish tank) n. 1 the Control Room, prison officials’ office, or guard house [...] fish box n. = fish bowl.