Hey bro,
First of all, don't add salt to your top-up water. Salt don't get evaporated from your aquarium, just H2O. If you start topping up with salt water, your tank salinity will start to rise.
Secondly, the NH3 rise may be due to micro-organism die offs in your aged water, or may even come from your dechlorinator. I suggest you start a fresh pail of aged water without adding dechlorinator nor salt. Then test it to compare results.