That should be 'a koi in a frozen pond'.
A koi is a Japanese carp which looks a bit like a foot-long goldfish (hence the second line in the song: 'like a goldfish in a bowl'). Kois are kept in garden ponds for their beauty.
It's not an idiomatic expression as much as a metaphor.
You can probably find a more poetic translation than mine. The question you have to ask yourself is whether you want to find a similar metaphor or explain its meaning in your translation. Personally, I think it could perhaps be interpreted as an image of the empty (frozen) feeling of a person who realises they're all glitter and no substance--like a decorative fish.

Hope this helps,
Mads