Water Tested, Nitrite 10.0 ppm Nitrate 40ppm Ammonia 0.50 HELP

I tested my pond water. I have Nitrite at 10.0 Nitrate at 40 and ph at
7.4 and alkalinity at 120 hardness at 120 Ammonia at 0.50. I am bringing
a water sample to my pet store. He checks it free. But Im worried. The
fish seem ok. But the small baby 1 inch fish are dying. I thought it was
lack of oxygen because I dont have the waterfall installed yet. The fish
seem happy. Last night they seem to gulp at the surface for air. But
then in the morning they are hiding under a built flat rock on top of a
pillar. I did a 20% water change after the reading. Im worried The water
is toxic. The pond is 3 weeks old. Thank you