quick question about Water..

i have gallon containers of water (from sink) sitting out in the open with your ordinary fish tank air pump inside them to enrich the water with oxygen after sitting out for 24hrs i do a pH test and it reads 7.0 so i add pH Down to get the Lv. to 6.0

NOW HERE IS MY QUESTION

the next day i go to test the water and its back at 7.0 but if i add the propper amount that i usually put to make the pH 6.0 it becomes to acidic, if i shake it up to get a better read on the pH nothing happens. so basicly what im asking is

DOES THE PH OF WATER CHANGE ON A DAILY BASIS? AND HOW DO YOU MAINTAIN THE WATER TO STAY AT 6.0
 
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