Muriatic acid?

[Miami Reef]

The reaction is instantaneous. You can test immediately. It’s a good idea to stir the water for a few minutes to have it homogenous.

I apologize for not fully elaborating. You should probably wait a few minutes of continuous stirring to get the alkalinity into equilibrium to get accurate testing results.

It’s akin to hotspots when boiling a baby bottle in a microwave. Some areas will be too hot and others will be cooler.

However, you will definitely need to wait 18-24 hours IMO to bring up the pH within healthy ranges. Too low will dissolve your corals or harm your fish/inverts.