How do you get water circulation in the tank? With a canister filter outflow? With powerheads? or what? CO2 that measures out to 30 ppm at the glass at a certain height may not mean there is that much amoung all of the plant mass. We rely on good water circulation to all areas of the tank to get the CO2 delivered to all of the plants.

You shouldn't be able to measure any ammonia in the water. Either the test isn't quite correct or you have a bigger than usual source of ammonia in the tank, such as a big dead snail.