This is a dangerous direction to go - it is nearly always best to adapt your fish to your water than your water to your fish - but it can be done. A Reverse Osmosis gizmo allows you to reconstitute water to your own specs or mix with tap to reduce pH.
If you decide you must lower your pH and do not want to go the RO route, here is what I did. First, I put lots of peat into a second canister filter. This managed to bring the tank down to 7.0. So far, so good. But not far enough, so I bought a gallon of muriatic acid from the paint section of a big box store. This is nasty toxic hydrochloric acid that eats through carpets if just one drop gets on it <ahem>, so wear safety glasses and gloves when handling it. By experimentation I learned that I needed 10 ml of acid (I used an old medicine dropper to measure) to lower 20 gallons of tap water from 7.6 to under 6.0. Do this in a separate container at least two days before mixing the aged water into your tank. The reason for this separate container and not just adding acid to the tank is this: the acid immediately drops the pH like a stone but as the buffers in your harder water chemically interact with the acid, pH rises back up. These pH shifts can kill fish. The right amount of acid "uses up" the buffers to just the right degree, so you must experiment to learn how much to use. It was not much in my case. Agitate the water as it ages. Use it for your next water change.