def run_one_day(fish: list[int], start_time: int = 6, new_time: int = 8):
    """Runs one day of lanternfish reproducing."""
    fishes = fish.copy()

    for i, f in enumerate(fish):
        if f == 0:
            fishes[i] = start_time
            fishes.append(new_time)
        else:
            fishes[i] = f - 1

    return fishes