Green’s Dictionary of Slang

fish-eye n.2

[supposed resemblance]

1. (US Und.) a diamond.

[US]Jackson & Hellyer Vocab. Criminal Sl.

2. (US milit./prison) in pl., tapioca (pudding).

[US]N.Y. Herald Trib. 27 Apr. 20/2: Here is a list of navy ‘slanguage’: [...] Fish eyes—Tapioca pudding.
[US]Charleston (WV) Daily Mail 2 Sept. 6/4: Psychologist James Hargan of Sing Sing prison has collected slang phrases from prisoners [...] ‘Fish eyes and mud’ – chocolate tapioca.
[US]N. Algren Man with the Golden Arm 208: ‘Rubber heels and fisheyes again’ was the word on the meatloaf and tapioca.