Let me explain the buffering system, this example is used quite often. Buffering capacity is the water's ability to keep the pH stable if acids or bases are added, usually refered to as your carbonate system(dKH). The higher the dKH the more buffer is in the water. If the water has sufficient buffering capacity, the buffering capacity will absorb and neutralize the added acid without significantly changing the pH, so think of the buffering capacity as a large sponge, as acid is produced, the sponge' absorbs the acid without changing the pH much. Howver if the sponge's capacity is limited it will not be able to soak up the acids being produced in the water, once the buffering capacity is used up, the pH changes more rapidly, hence pH crashes.
Ok am I right in assuming that your tanks dKH is about 1-2 dKH,assuming that water usually has a Co2 dissolved at about 3-4ppm. Your tank probably does not have what is called a high buffering capacity, but should be fine if you do regular maintenance. For your tanks pH to drop overnght there would have to be a lot of acid being produced by the wastes that your fish are producing and you not replacing the buffers by not doing water changes. Adding crushed coral will increase the buffers and if you feel that you want more buffers by all means do so, but your pH will then rise. Anyway I hope that answers your questions.
And Kas fish wastes have a big effect on pH.