First was that a typo on the salinity? Did you mean 1.023-1.024 or is it really 1.23-1.24? Should be around 1.025-1.026. If it is as you typed it, that is what is killing the fish. If it was a typo, I don't think that it is the killer but from what I have learned it should be a bit higher.
Any marks on the fish after they die? If it were a critter getting them I would suspect torn fins, red gills, something like that. What test kits are you using? I use API but after buying them I found out they are not the best. Could be you think you have good readings but something isn't test right. Have you matched your readings by one done by a LFS
out of the same water sample?