Nope, filters won't do it.

Filters can remove salts and minerals that affect the pH of water. The pH of highly purified water is hard to measure, due to the lack of ions present. You could buffer your water with the addition of low salt concentrations to obtain the desired pH. I am guessing that you are looking to obtain a neutral pH and so the addition of phosphates could help.

Thanks, chervil2. So am I right to think that a water filter would not DIRECTLY lower the pH, but by removing salts and minerals it would have the indirect consequence of lowering it? I think you're saying that I could EITHER use a a filter OR add low salt concentrations, and that either of these could/would have the effect of lowering the pH?

Or ... do I need to do both? I bought a product at a hydroponics store called PH Down and then called the manufacturer with advice on how to use it. When I told him of my 9.1 pH, he said that just using the solution would only work temporarily, that after a day or so the pH would most likely float right back up because of the probable high level of total dissolved solids. He said that I would have to first use a filter to remove the salts and minerals, and THEN the Ph Down would then be able to permanently lower the pH. Does that sound about right to you?

I'm pretty sure my water has a high content of sodium carbonate.

btw, what does it mean to "buffer" my water?

Thanks much for your help on this -- this is all learn-as-you-go for me.