First ensure your newly mixed salt water is at the correct SG/Salinity (1.026 / 36ppt).
Next check the salt mix parameters to the specs provided by the manufacturer as the mix values. Common ones found here:
https://www.thatpetplace.com/salt-mix-guide
Third if you do not have coral in the tank you shouldn't have consumption of Alk, Cal, & Mag unless you've got crazy coralline algae growth (the good colorful flat stuff that makes rocks purple in most cases).
I suggest testing for ammonia to rule that out as a cause of death. If all the dying fish are coming from the same source, try another source. Set up a quarantine tank if you can. One completely separate from the main to ensure no cross contamination.
Lastly you could be just over crowding the tank and/or not providing sufficient places for them to hide. Search online for the compatibility chart. Although not 100% accurate, you'll get an idea of which species will have the best of luck together.
Edit: I browsed the previous two threads. I suggest slowing down and allowing what you currently have in the tank to settle for a while. When you do decide to introduce new fish, do so slowly (one or two at a time max).