Honestly, in most cases you do not need to alter your PH if you are simply keeping fish.
Maybe in some cases you do if the fish only breed in say soft acidic water or the hard water would be harmful to them. I think that is rare and most fish will be fine in your water.
My water is very akaline and the total hardness is off the charts. I never mess with it.
Your LFS probebly has the same water as you do or close. Even still, properly acclimate the fish into your water and they should be fine.
Dont use PH up or down or any PH stabilizer. Your hardness is your stabilizer and why it is taking so much to move the ph.
Carbonic acid will lower your PH temporarily from Co2. It less of a swing when you have a lot of hardness in the water compared to softer water.
If you are looking for soft water for a specific reason, may be easier to use R/O water and mix it with yours for the desired PH and softness/hardness.