You posted like many days ago you added new tank mates, where they from a LFS or from your other tanks?
How long has this tank been running and is there enough bio load to cycle a 200G tank?
My friend went through 2 test kits because one was given a false reading. He had detectable ammonia ended up killing some expensive fw rays and added the live bacteria for ponds (cheaper than aquarium ones) and it gradually reduced it to undetectable in a few days.
What does the dead fish look like, any distress in the gills and is their fins intact?