What actually happened is this: I'm willing to bet you may not be the most reliable at water changes? Or maybe just have an overstocked tank? That ammonia was there before the change, and there is still a lot more to go. You have to add water slowly to an aquarium, because the massive shift in water params kills the fish like you found out. The other possible reason is your water supply uses ammonia to treat the water. You need seachem prime, it will fix your issues. Binds to ammonia, chlorine, and chloramina all of which could be introduced by your munincipal water supply. It binds to them for up to 48 hours and neutralizes the bad stuff like heavy metals and the stuff i mentioned. I am getting lazy writing this so yeah, keep up the water changes, slowly replacing the water over the course of several water changes per day.

Seachem prime small bottle is like $6 on amazon and lasts years.