Do you know your tank water hardness and or TDS
Do top off your water consistently with Tap water or RO
How much water do you change on a regular basis? Size of tank?
Do you notice any of these deaths before regular maintenance, after water changes?
How often do you do pwc
and how much do you usually change?
Were these adult fish when you got them?
You have BB in your filter pad, don't change the filter pads?