Fin nipping!? what should i do!?

i have a well planted 120 litre aquarium with 1 angel fish, 2 rainbow fish , a honey gourami and 2 serpae tetras from the start 1 of the serpae tetra who has abit more vibrant red colour constantly nipped the other one ching it around constantly looking for it all the time to har it this has been going on for about 2 months now the tank has alot of plants and ornamens to hie in but the other one always finds it it! the innocent one seem to be swimming on its side randomly sometimes what should i do?