Hopefully there's not much of a swing when you do water changes. It may be better to do a few smaller changes during the week instead of one big one. All this talk about the importance of a stable pH is great and all, but if your water is going from 7 to 8.5 then your pH isn't all that stable.
At any rate, pH doesn't increase via magic. Something is causing a significant change.
Id start by double checking your tap water. Before testing , let it set in a container over night and either leave an air stone running or give it a good stirring from time to time. This should give you a more accurate reading. In fact, it'll probably be lower than the reading you get straight from the tap.
Sent from my iPhone using Aquarium Advice