I would not attempt to change the ph of the water with any chemicals. If the fish is acclimated to the ph or your tap water leave it alone. I would test your tap water and see what the ph is out of the tap. As far as frequency and volume of water changes, let the numbers be your guide. As long as you are keeping the nitrates below 20ppm you should be ok. This may require 2x wc a week or one a week, there are a lot of variables, stocking level, feedings, filtration (bio-mass capacity) etc. Test weekly and you will see what your tank needs. After a while you’ll know how much and how often. GL