I've been noticing all this talk of buffers crashing and I'd like to note that my tap runs water that's something like pH of 7.8 or so and I've been able to maintain a pH of 6.5 for my betta and neons by adding pH lowering drops with each water change. Is this not a reasonable solution to his problem? I've never had my pH randomly rise again just because I didn't add new droplets...
Also, I'm thinking maybe it's something in your water. I know our well tap at my old house hald a lot of rust in it and wasn't really healthy for fish, even though the tests said it was. We had a gravel company behind our house blasting rock out of the mountains. It caused all kinds of sediments to leech into our water. Once we found out, we didn't even drink it!! O__O;;