Are you using tap water for pwc, and what is the ph of that?
I'm sure others will have loads of insight on this but meanwhile I found this advice (haven't tried it myself):
"That pH does sound pretty high. It's good that you haven't been using chemicals to lower it though. They cause fluctuations that can be very stressful to the fish and don't work long term in most cases. You can add driftwood or bogwood to your tank soften the water instead. What some hobbyists do is use "peat water". Add some peat pellets, available in fish stores, to a pot of water (Not aluminum). Bring the water to a boil and then put the "tea" with the peat into a brand new clean plastic garbage bin full of water. Keep it covered and full all the time. The peat will acidify the water slowly. Use partly peat water and partly tap water to refill your tank for water changes. You will need to test the pH and hardness of the peat water to decide how much to use at a time. Peat may color the water brownish or yellowish. Fresh carbon in the filter will help."
Lots of people here don't do the carbon.
Here is the rest of the reply:
"As your aquariums system ages the water will naturally become softer too. The organic wastes made by the fish will acidify it over time. Salt isn't going to hurt the fish or anything but it really isn't going to soften the water either. The minerals must be removed from the water to soften it and lower the pH. Salt works in water softener machines to "recharge" the resin on a regular basis. It's the recharged resin, not the salt, that softens the water." Quoted from another forum