Thanks Sandy. I don't think the ammonia killed her. I'm just thinking it's the chronic non-lethal exposure to it over a few weeks which weakened her immune system so this bug, whatever it was, could take a hold. It was a very unpleasant experience watching an otherwise normal and healthy looking animal, knowing and just waiting for her to die from something inside her that I couldn't do anything about. I definitely don't want to go through that again.

Annette, I understand what you're saying. I had a look at Seachem's website on the
Prime FAQ. The Prime 'binds' to the ammonia to detoxify it. The Kent Marine label says it does the same thing.
The Seachem FAQ does say 'salicylate' tests like our Aquarium Pharmaceuticals ones, you should take the measurement straight away because the five minute wait is long enough to break the Prime/ammonia bond, 'eventually giving a false reading of ammonia'. Which I take to mean it should read no ammonia at first, but then slowly turns green. But my post-Prime tests yesterday turned green immediately. I didn't even need to wait 10 seconds, let alone 5 minutes.
I did a quick test with 3 drops of Prime in 5ml test sample of tapwater (a large overdose of Prime!) so I can see for sure if the ammonia still registers even when it's 100% detoxified as you suggest. On the left is with the 3 drops of Prime. On the right is fresh tapwater:
No ammonia registering with the Prime'd sample, so it's definitely detoxed. Granted, 3 drops per 5mls is much, much more than the instructions say or what I was dosing yesterday, but I wasn't getting anything near like this yesterday, even with as much as 5mls per 25 litres. Just pure green from the outset.
It took 5mls of Kent Marine 'Ammonia-Detox' in 25 litres to register a yellow colour. As it's about 70% stronger than Prime, I guess the equivalent amount of Prime needed would be about 9mls per 25 litres. About 35 times the instructions!
I'm concluding I now have 0.5 - 0.7ppm of pure ammonia in my tapwater, as well as an extremely high amount of chloramines which I think Prime breaks down into ammonia, which is what I'm still reading after adding Prime. Or maybe I'm even reading the original 0.5 ammonia all along, while the Prime is being used up on all the chloirine and chloramines instead. I don't know enough about it. If I'm wrong, please tell me, as these ammonia detoxifiers are very expensive!
PS. I've always had the opposite experience, Annette. With the exception of the last few weeks (which has been the exact opposite), my fish always seem to perk up dramatically after a large water change. Even today when I did a 60% change (now I'm using Kent Marine Ammonia-Detox, so no ammonia!), Bumpy came to life, zipping about the tank like his old usual boisterous little self. Which I'm very happy to see!