Goldfish are a symbol for luck and prosperity in Far East Asian countries (especially China and Japan). There are usually 8 golden ones and 1 black one. The black one is put in there to absorb negativity and any bad luck. So it stands for protection. If the black fish dies, it means it absorbed bad luck and negativity that was coming your way. It is a sacrifice.
Although the tank does not keep 8 goldfish, there is ONE black one mixed in and it is dying. Mo wants the black one that is dying. It ties in with everything we’ve seen so far. He thinks he doesn’t deserve happiness, he considers himself a burden if he cannot support his mother. Because he is his mother’s protection. He absorbs all the negativity, he always takes the fall for everything and everyone. He is the black fish.
