My koi's are dying and the water levels for ph are right- what am I doing wrong ? I change the water every week and feed them twice a day- am I doing something wrong?

Did you test the ammonia and nitrite levels?? Are there any signs of stress, gasping for air at the surface? What are the total gallons of water in the pond? How many koi and what are there size? Water temp. also, where do you live(state)?