Sorry to hear about your loss. They are such beautiful fish.
I've had a similar dilemma.
I've had 5 Black Congo, and 5 Inkfin for about 4 months now. They were roughly a little over 1 inch when I got them.
Every time I do the water change, 1/3 of 75 gallon tank, all of them but 1 good size male Black Congo would lay down on the bottom of the tank almost motionless for an hour or so.
After that they all seem to revive one by one. The smaller ones are taking more time than the bigger ones.
The one which is not affected by the water change, is acting normally like the rest of the fish in the tank (Lamprologus Leleupi, cylindricus, tret ...).
That whole ordeal makes me scare to change the water. Just like you, I use Aquachem to detox, and the temp is pretty closed to the tank's temp.
Anybody know: Would they get better when they get bigger?
What else one can do to make the water change less harmful to Calvus?
So far, fortunately for me that none is dead yet, but it's just like playing Russian Roulette. Reading your post scare the crap out of me.
I hope one day you would find the strength to get them and try again.
SM