Corals make great testers.I agree hobby grade needs to be taken with a grain of salt. I will try to get the result confirmed this week. I think another Hanna checker is in my future.
In general, ultra low or zero nutrients will result in poor polyp extension and drab coloration, or paleness
High nitrate may causes browning or stunted growth.
That tank pic did look good to me.
Bacteria bloom thing is weird, only seen that with an overdose of carbon like Nopox or Vodka