RO water (though expensive) is an option, but it is basically the softest water you can get. You have to actually give it a buffer and dial it in to suit your needs. With it, you wouldn't need to do a water change each day though. In fact if you did that you would have worse problems then just the 9.0 pH. You would get all kinds of ups and downs as you tried to keep the water you were adding each day the same as what was in your tank.
Speaking of water hardness... have you done a test to see how hard your water is? If it's KH is really high, then it's possible that is why you can't get the pH to move even with an acid buffer. I would see what your
GH and KH are and then go from there.
Vin... aeration will normally raise the pH when it's removing excess CO2. Properly aerated water (usually our tank water will be aerated enough just by the flow of the filter especially with a
HOB filter) won't have enough excess CO2 to cause the pH to rise. Which is probably why you never had an issue with your tank.