You are still in early stage cycle and although you achieved zero reading, the tank with bioload (feeding, adding fish) has potential to rise again as ammonia is the natural waste product of fish. With ammonia and nitrate you can even have occasional spikes and until the anerobic bacteria multiplies to point that is can digest waste faster than produced- you will have ammonia present hence reason for water changes. The bacteria population will increase with new bioloads, processing waste and converting it to nitrate. After a feeding , it is Not unusual for ammonia level to increase as much as .2ppm and then gradually drop again. The bacterial levels will adapt if you don't overload the system with too many mouths to feed and realize the water quality I mention can be yours or that of the LFS where you got the fish from. Ammonia is the natural waste product of fish