Me, Me, Me!
Back when I was working at a store, we had one crash a small display tank after getting sucked in by a powerhead. It happened FAST, <5 minutes. Two tangs, several shrimp and a flame angel down the drain - poor guys
To answer your other question, no. NH4 readings would be irrelevant due to the speed and severity of the toxin.
Don't put one in your tank unless you're dedicating a tank to it.