The crushed coral is raising 2 things, pH and KH. KH is going to buffer your water against pH changes. Meaning that if your pH is for example 7.5 and you do a 50% water change with water that is 5.5 your pH won't change to much lower than say 7.0 if at all.
Elevated KH prevents the pH from changing by absorbing either H+ or OH- ions.
Elevated KH prevents the pH from changing by absorbing either H+ or OH- ions.
