I don't know. However, these corys like pH 5.8-7.0 so 7.5 is pretty far out of their range. I expect it stressed them if not being the sole cause of their demise.
What stresses them is sudden shifts in pH which, when you use products like pH up or pH down, is exactly what happens. Most people use those products without understanding exactly what it takes to change the pH value, I use to. The pH value of water is the resulting factor of the relationship between gH and kH, and could be seen as such; gH+kH=pH, though that isn't entirely accurate. The kH has more influence on pH than anything.
Of the aquarium fishes we keep in this hobby most, not all, but most of them are tank raised and can handle the higher pH ranges typically found in our tanks. Most likely the fish you buy around you can from tanks in your state, as I believe the two largest distributors of ornamental aquarium fish here in the U.S are in California and Florida.
I regularly checked for amonia and it was always zero. I cycled these tanks sometime ago with the amonia method and Tetra SafeStart.
But unless you've kept the bio filter alive some how AFTER you cycled the tank, all of the bacteria die off if they have nothing to eat.
When ever I increase to bio-load on a tank I always use the safe start for a few days to a week.
Thanks. You think angels will survive in a pH 7.5 environment?
Angles can, unless your dealing with Discus and shrimp most other fish will fair just fine in a wide variety of pH ranges. When you start importing wild caught fish it's a different ball game.
Now that I've responded to all of that.... My last order from LA was 3 False Julies (which were labeled wrong) and 4 otos, EVERY FISH died, EVERY ONE. Of the 16 fish I've purchased from them over the last year only 5 are still alive. Their fish just don't do well for me at a 75% mortality rate I think I'll buy fish from somewhere else