Well, it might be a metaphor for the fish's unconventional dreams or a way to add an element of fantasy and contrast to the story. It could also show the fish's rebellious nature against its normal environment.
It could be a symbol of longing for something impossible or out of its element. Maybe it represents a desire for a completely different and unexpected experience.
Perhaps it's a creative device to explore themes of contrast and aspiration. The tropical fish yearning for snow could be a way to highlight the fish's unique and perhaps unfulfilled desires, adding depth and intrigue to the yuri manga.