Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I would get another ammonia test and compare results, or else get a bottle of ammonia and add a few drops to some water that you KNOW has none, perhaps get a bottle of reverse osmosis water.
See if there is a problem with your water.
That low pH would keep the ammonia in the ammonium form. Not exactly non-toxic, but not as deadly as ammonia. Still, that would be enough to stress the new Cardinals.