Welcome to Tropical Fish Keeping forum.
One thing I didn't see mentioned in this thread concerns your test for ammonia. Prime detoxifies ammonia by changing it to ammonium which is basically harmless. However, the API ammonia test will read ammonia and ammonium as "ammonia," so after a water change you might still see "ammonia" with a test but it is actually ammonium.
Live plants need ammonium as their source of nitrogen, so that is why adding the plants as others recommended has worked. Prime will detoxify the initial influx of ammonia at a water change, and by the time Prime wears off (24-36 hours) the plants will be assimilating it.