IF you are using the same water AND it tests the same then obviously that isn't it. But if you think that just cause you both use tap water that the water's the same then you can be sadly mistaken. A neglected tank may have conductivity higher then 2000, while a well maintained tank may have the same as tap (could be low as 200, high as 800 -all depends on your area ).
All you can do is rule out possibilities. I didn't go buy a conductivity meter cause I had a pocket full of extra money, I bought one to rule out the possibility that I was shocking my fish. I got tired of grasping for explanations so instead of blaming it on something I didn't monitor, I started monitoring it so I KNEW that it could likely or unlikely be the cause.
IF it was this kind of shock the fix is simple, match the numbers. Dunk the probe where the ATF is happy at, and match your tank number to that number. I have really hard water. I use RO to bring conductivity down. IF I needed to bring it up I'd add tap water or buffer with aragonite.
Like I said all you can do is rule out possibilities. I'm not saying this is what is killing your fish, but if you don't know, it's not ruled out, and all we can do as hobbyists is try our best to rule things out, monitoring the numbers can usually rule stuff out. Most unknown deaths get lumped into a category like PH shock, this way maybe you could at least rule that out.