KOI ARP BEING BULLIED BY GOLD FISH & CATFISH?

I have a 12inch longfin koi carp in a 300litre tank together with 4 comet goldfish approx 6inches long & a 2inch common goldfish. Oh & one common goldfish 2inches long. The issue is my Koi is being bullied by all other tankmates, especially the pleco catfish & the 2inch common goldfish. I have temporarily removed the koi carp from the tank to give him peace. This started recently after i reduced food rations following an acute ammonia poisoning suffered recently, (they are still recovering). Am afraid of losing my Koi carp due to stress. How can i contain the situation? Please please ist.