I'd consider fish gasping from no2 poisoning an emergency.
Fish Disease - Nitrite Poisoning
Did you happen to read the link that you posted?
None of the symptoms are noticeable in that video
Fish gasp for breath at the water surface
Fish hang near water outlets
Fish is listless
Tan or brown gills
Rapid gill movement
Maybe a video with a closer up look at the fish would reveal visual symptoms of poisoning, but those fish didn't seem to be listless at all, or breathing heavily.
You assume wrong, since it doesn't carry over from ammo to no2.
"To detoxify nitrite in an emergency, up to 5 times normal dose may be used."
Okay, tell me then how much nitrite Prime detoxifies in a regular dosage? It seems like even the seachem techs can't give an exact answer to that one.
I know what the Seachem site says about overdosing Prime, but a .50 nitrite reading (before water change) is far from an emergency. Although I guess that would be a matter of opinion.
Another point that could brought up is the toxicity level of nitrite in basic water. The hard water generally found in an african setup helps ensure a low level of nitrite toxicity, as no2- is far less toxic than hno2.
If anyone cares to get their learn on, reference Chen, J.C., Cheng, S.Y., 1999. Recovery of Penaeus monodon from functional anaemia after exposure to sublethal concentration of nitrite at different pH levels. Aquatic Toxicol. 50, 73-83