So your saying that your other results of "ammonia 0 nitrites .25 nitrates 20" came after the die off? Again, I really don't see how that's possible, and if these were your results after the die off, you need to figure out why you are/were getting false 'zero' readings.
Chemicals like Prime will, as far as i know, give you a false positive. What you're looking for is why you got a false 'zero'. I actually have no idea what chemical, or anything else, that would give a false 'zero' reading, other than maybe a defective test kit, which you've already said isn't the case.
I do hope you figure it out, and don't just "chalk it up to a loss". Some problems can't be figured out, those problems are rare. Most problems we do figure out with some effort, so we don't make the same mistake twice. That information goes into places like MFK so that others may never make that mistake at all. and that is what makes knowledgeable (responsible) fishkeepers.
Good luck,