Your fish died because you did not acclimate them correctly. Look up drip acclimation. It takes several hours & if the tds is more than say 40 TDS different they should go to another tank anyway for quarantine so not to spread disease. All new fish should go to Quarantine so that you can adjust the kh & gh & TDS slowly & you can watch for signs of disease, so you do not spread it to healthy fish. But they need to be drip acclimated to the quarantine tank as well & re dripped 30 days later to their final home. Stressful yes, but human as possible to the fish.