If you have live plants, add in CO2 through a DIY system (or even pressurized if you can afford it!) The CO2 will not only help the plants, but it will naturally lower the PH as well. My PH is normally around 7.0, but with CO2 addition to mine gets to 6.2 or so.
Also, filtering through peat, addition of driftwoods, and chemical stuff will lower PH as well. But I am guessing your tap water is that harsh (probably hard and a high kH as well ...) so you might want to go with bottled water or RO water or even rainwater ...
I don't think there is a whole lot you can do without a huge investment, sadly. Hopefully some others will be able to comment!