Fish Polka is a hidden musical composition in Rise of the Triad.  You can hear
it by starting the game with the "dopefish" command line parameter or by
bringing up the jukebox in the game (see the cheats section).

Lee Jackson's paraphrased history of the Fish Polka follows:

When I originally wrote it, it was a measure shorter than the final version.
The four count intro didn't exist: the song started right in on the melody.
Mark Dochtermann, one of the game's programmers, suggested that I take a note
from the Schmenge Brothers (from Second City Television (SCTV)).  They were a
polka act, with John Candy and Eugene Levy playing Josh and Stan Schmenge.
Mark said that all of the polkas they played started off with the same goofy
four count intro, which he then sort of sang to me.  He said that I should
include something like this at the beginning of Fish Polka.  The next day,
my homage to that intro was in place, and when I played it for Mark, I got
the only high five I've ever got from any of the programmers here.  Now I
can't imagine the Fish Polka without the intro.