I wonder what your test kit would show about their tank water - you didn't by any chance test the water in the bag
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I have good quality strips stored properly, which I don't use, but reading your post I gave them ago,they seemed OK but,when they came to nitrate, I think they were off, on my two tanks one was 40, the other 80, I then used my NTLabs liquid test, and nitrate came up zero, on both tanks I have quite a few live plants in both tanks, both tests were done properly.It is a myth that test strips aren't accurate. They degrade over time, but I have done tests with both side by side and gotten the same results consistently. Strips are expensive and you can't keep them anywhere damp, but they are accurate if used properly, and stored properly. Those are fair "ifs".
At my lfs, they demand a time stamped photo of the dead fish in the bag. Once it's in the tank, it's your problem.