pH falls sharply, often overnight, to well below its usual value.
Short answer
A pH crash means the carbonate buffer ran out. Test KH — if it is at or near zero, that is the whole explanation, and the fix is to restore buffer gradually rather than to add a pH-up product.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: KH at or near zero after a long period without water changes.
How to tell: Planted tank with injection; pH falls during the photoperiod and recovers overnight.
How to tell: Accompanied by an ammonia reading and often a smell. A dead fish, a large plant melt, or a filter left unrinsed.
How to tell: Aquasoil substrate, heavy leaf litter or peat, with near-zero KH source water. Often intentional in blackwater setups.