My supplier quarantine fishes for me and they keep them at 20 ppt. Their advice is to drip for 4 hours to match my tank salinity which is 34.5 and they are all fine swimming in my tanks. Fish can handle salinity jump if done correctly.Sounds like classic ammonia poisoning or salinity shock. Ammonia is usually from too long in the bag acclimating. Ammonia build up in the bag burns gill cells and it is downhill from there, Symptoms often present a day or so after introduction w/ death shortly after. Most common is shipped fish though where they've been bagged for a day or two.
For salinity, most fish stores run really low--like 20-30 ppt. Fish can handle a sudden drop in salinity okay but a sudden rise is deadly. They can really only handle a jump of 2ppt per day or so. Going from fish store salinity to your tank's salinity would have required several days of slowly raising the salinity (I do this by evaporation). Fish stores never tell you this when they discuss acclimation. Always ask the salinity and, better yet, test yourself.
Another good reason for a quarantine tank/container. Allows you to match salinity and then raise over time slowly. Much less stress on the fish to match salinities and release immediately.
Sorry for your loss. They looked like fine fish.