There are a couple of possibilities here without knowing more of your water parameters. If your pH has been inching up slowly, your fishes should be able to acclimate to the higher pH within a reasonable range and avoid osmotic shock. What is the exact value of the pH?
Secondly, if they are suffering from bloat, this could be a protozoan infection, and the pH might be a distraction. If the deaths have started over a gradual period of time, and is slowly picking off your fish, this would tend to support the protozoan or bacterial hypothesis, as general water conditions would tend to kill off your fish much faster and more all at once.
At any rate, crushed coral for a freshwater aquarium is really dicey, and only good if you have a naturally low pH in your source water. Did you add crushed coral to manage a low pH factor, or because someone sold it to you as a better alternative?
I would guess you are dealing with infection. If it was a pH thing, you're stuck between a rock and a hard place, since jacking down the pH quickly would be just as dangerous and damaging. I can't tell you what to do, but you have two options to start troubleshooting and eliminating possible causes through the process of elimination.