It may be the water, or it may be something in the tank (a parasite, or something.) If it is something in the tank, the frys should be first to succumb. Since they are doing well, that would make disease in the tank less likely.
Drip acclimatization like you did should do the trick, even with drastically different water. The only thing I can see that may be a problem is the salt. Did you measure the salt content in the tank? Or is this your calculated salt level. I am thinking this may be possibly salt creep. <Some of the water had evaporated, concentrating the salt level. If you don't account for that in your pwc, the salt level will slowly creep up over the course of a few months. The current occupants is OK because of the slow increase, but new occupants cannot handle it.>
you may have to set up a QT tank to solve this mystery. <QT is a good idea in any case.> If you have a spare tank/tub, you can set it up to mimic your existing tank's water exactly. (But don't contaminate it with anything from the main tank - water, substrate, nets, etc.) Put your new fish in it after acclimatization & see if they fare any better. <Normally in a QT, you would want the QT to be cycled by transfering some of the media from your main tank to the QT. But since you don't know if the main is "safe", you can't do that. You will have to do daily water changes to keep levels down, or possibly get the media from your friend's tank.>
If the fish survives QT (gives it a week or 2), but die in your tank, then you have ruled out the water parameter shift as the culprit. This may seem a lot of work, but QTing new fish is essential in preventing introducing diseases into your main tank once established. So you may as well get into the habit.
