The typical number for Koi fish in a feng shui aquarium is 9 (8 brightly colored koi and 1 black koi fish). The black fish is there to neutralize bad luck. You can also find feng shui cures with 2 koi fish, this being a feng shui cure to attract love and fidelity. There are also paintings with any number of koi fish in a stream, which is used to represent a fresh flow of abundance. Still, best numbers are considered either 9 or multiples of 9.