A gradual swing in pH will not kill a fish. There have been studies where fish are exposed, long term, to low pH (7.5 or lower) and it has been found it affects the blood proteins where it takes more energy for the fish to maintain proper blood chemistry. It doesn't kill them.
You can use crushed coral to buffer your pH.
Simply put, that's not true in a typical saltwater tank.
I think I advised you on this before, but once you start dumping in buffers with borate (boron), which is what any buffer has that says it keeps your pH at a certain level, your alkalinity test kit won't work properly (plus your pH will not stay at that level for long anyway). The boron will interfere with the test kit readings.