When you say your tap water is 8.5....is that a measurement of the tap water right out of the tank? Or is that a measurement of the tank water's pH after it's been in the tank for 2-3 days. I had a similar problem when I set up my tank. I found out, pH of tap water is meaningless. If I take tap water, measure it right outta the tap, I get high pH values, too--8.4-8.6. If I let that tap water just sit in a glass for a day or two, pH measure 8.0-8.2. If I do a 75% WC on my tank with that tap water, and measure pH of the tank water a couple days later, I measure 7.6-7.8. When I added driftwood to my tank, that dropped my pH by 0.2-0.4.
I would highly recommend that you run a freshwater tank with the tap water you have. Playing around with kH, monitoring TDS, pH up and down, that's a lot of time and effort. If you slip, or make a mistake with all that, you get a tank of dead fish. Not saying it can't be done, but I personally wouldn't do it. If you really want to do it, though, read up about marine tanks and how they manage all these variables. They use RO water exclusively, and they monitor all this stuff. However, I believe it's easier for them since the sea salt they add to the RO water has all kinds of buffers in it to stabilize pH. Since you wouldn't be adding salt, you'd need to find some things to add to RO water to make it appropriate for a FW tank.