Hello, what is the pH, ammonia, nitrite, and nitrate readings of your water at the times around the fish deaths?

I suspect 1 died and the ammonia got out of control because it was left in there too long. The evidence may be "out with the bathwater" given all those water changes.

As mentioned previously by David, the unionized ammonia in the water increases with the pH. However, the toxicity of unionized ammonia does not increase, this remains constant. What increases is the respective proportion of speciated unionized ammonia in aqueous solution. In other words, you have the same total amount of ammoniacal nitrogen (ammonia/ammonium), but the balance of lethal ammonia to ammonium tips towards the more toxic version of the molecule in water; thus, killing the fish.