I hope this is not too late to help. You could have a major issue here and lose the fish, as I'll explain.
First, in acidic water (pH below 7) ammonia always changes into ammonium immediately, always. Plants have nothing to do with this. Ammonium is harmless to fish. So as long as the pH remains acidic, fish are fine (with respect to the ammonia/ammonium issue only).
If your tap water is basic, pH above 7, you must do very small water changes to restore the tank pH to where it should be for the fish. Very small. If you do a major water change and the tank pH rises above 7, as someone mentioned, the ammonium immediately changes back to ammonia, and bang go the fish. Using a water conditioner that detoxifies ammonia will help, Prime is one but several do; they work the same way, by changing ammonia to ammonium. But this can have its problem too, because Prime and other conditioners only work for 24-48 hours, then the ammonia returns if the bacteria has not multiplied sufficient to handle it.
Nitrifying bacteria also die off in acidic water. Some authors suggest 6.4 is the pH below which nitrosomonas bacteria cannot multiply, some suggest 6 as the pH below which they will simply die.
There is also the issue of major shock from too quick a pH adjustment, but that is secondary to the ammonia issue.
And members were correct on the ammonia/ammonium reading the same on test kits; most of those we use, like the API, do not distinguish between the two forms and will show "ammonia" whether it be ammonia or ammonium.
Whenever a tank has reached this state of neglect, bringing it back must be done slowly.