A couple of issues.
First, your undergravel filter. Any chance it is clogged, or there may be something under the plate? Decaying anything will increase ammonia--dead snails, fish, plant leaves, detritus, organics.
With the pH below 7 the ammonia will be harmless ammonium. Test kits read ammonia/ammonium as ammonia, so if the pH is below 7 it is not an emergency, but the cause should still be ascertained since this is not normal or healthy.
Plants consume considerable ammonium (ammonia) so again with live plants having ammonia/ammonium showing on a test kit is cause for concern as it never should.
Is there any chance the pH may rise above 7? If the ammonia (as ammonium) is really 8ppm and the pH rises above 7, I can promise all fish in the tank will die quickly. Be careful with water changes if this could result (an increase in pH).
The Aqueon water conditioner will not handle ammonia, according to what I found on their website. This is only an issue if ammonia is present in the tap water of course, in which case a conditioner that detoxifies ammonia would be advisable for water changes.