After running water through a bottle of peat moss for a few hours, the pH was down to about 7.4, and another day of that didn't seem to make much difference.

It sure made some brown water though. I also tried hitting it with the water softener resin and some acid, but neither seemed to make a difference. However, 7.4 is better than 8.6, so I took out a gallon of water from my tank and have a gallon of this dripping in at a rate of about two drops per second, which should take about 12 hours.

I guess the best approach might be rain water, but I'll have to figure out how to collect it and then wait for it to rain.