Jordy AfricanCichlid expert...just crush coral gravel & u should be fine...
+1
Don't mess with chemical pH fixes. Your pH will never be constant, as every time you do a water change, you will expose your fish to a variety of pH ranges, and it is doubtful that you will consistently alter the water pH in the identical way. Frequent pH swings make for stressed/sick/dead fish.
But the PH is not the culprit here.
What species of cichlids are you keeping? 35 of any species, even dwarf cichlids, is far too many for your tank. They almost certainly died from stress related to overcrowding.
You should return or sell off the vast majority of that stock or you will be seeing more of the same. Its not fair to the fish. Not trying to sound abrasive or combatative here, but you definitely overestimated the number of fish that tank can comfortably house.