I'm so sorry to hear about your Hippo Tang.
That's so sad.
As far as your test kit goes, I use API for my freshwater, but I don't believe it's accurate enough for saltwater. I use RedSea and Salifert Tests (but like I said, beware of the Salifert Alk test). The only false reading I get on my RedSea is the Nitrite test, but I have a Salifert Test to back it up.
Your Kole Tang looks great in your tank.