I had some similar problems when I first started TT. Some of the guys here suggested that I change to using DT water. I did that and haven't lost a single fish yet. I did TT on 3 pyramid butterflies after failing twice before with this fish in a cycled QT.
Honestly I think the health of the fish on arrival is the most important thing and that's not always obvious when they first arrive. For me however, using DT water seemed to help. Don't know for sure if it was that or the quality of livestock but 7 fish in a row have made it safely through TT, observation (with Prazipro) and in to my DT.
Agreed. However, it seems there does seem to be an occasional issue using freshly made water based on replies to this thread. I also recall reading in other threads that during TTM there have been occasional sudden, unexplained downturns after transfers during TTM. Wonder if these are related to 'new saltwater syndrome' for lack of another term? Being a relative newbie and with certainly much less experience than everyone trying to help, I wonder if it happens frequently enough it to mention a warning as a posting in the main TTM thread? Or is just one of those pretty rare things that can happen, like probably an infinite amount of other potential problems? Maybe it only affects fish that are already on the 'edge' (i.e. the straw that broke the camel's back.)