Its not just outgassing that will keep or raise the PH in water. Dissolved minerals can buffer the PH up or down depending on whats in the water. Things like limestone, chalk, and especially calcium carbonates - from sea shells, etc, can buffer the water. They will let the PH drop initially, but cause it to go back up as soon as all the acid you added is used up. The chemical reaction takes a little time depending on the exact chemical nature, the exact acid used to drop PH, other things in the water, temperature, etc.
Ok, which part? 
I think it time for you to come clean! 