You don't need to change the pH, yours is fine - more of a pain to try to keep it regulated. Not to mention you will be removing all the beneficial minerals they need when you use bottled water.
A pH of 6 is good for your CT (judging by your avatar), harder water tends to curl their fins.
If you think you need to raise it (again, you have a fine pH and in the long run using bottled water can be detrimental to the fish's health) then you will want to remove him into his cup, and then after the water change float him in it, removing partial water from the cup and replacing it with water from the tank. Do that every 5-10 minutes for about 30-40 minutes so he adjusts to the new chemistry.