The way I've heard it for aquariums, a pH crash is either
a) excess CO2 into an unbuffered tank causes pH to drop fast
b) CO2 stops, pH rises quickly - again, as a result of having little or no buffering capacity
Having some buffering capacity will help prevent situation (b) from happening, but having the pH change fast enough to kill fish isn't likely anyway as the pH will rise gradually as the CO2 diffuses out of the water.
Situation a can only happen with a pressurized system and only when it fails in the on position, injecting lots of CO2 too quickly. Even in that situation, I din't know if saturating aquarium water w/ CO2 could bring the pH down to <5. Buffering capacity would help a little, but w/ unlimited CO2, the pH will still drop. If it drops quickly, fish will die (probably from lack of O2).