Download Bru'n water and start learning. That should get you started in the right direction. There is a learning curve to water and pH but it is very worthwhile IMO. Since you are already knowledgeable about yeast I figure you for a guy that would not be too intimidated by it. The short answer to your question is acid. You will likely need acid to lower both mash and sparge liquor as low as I stated. I prefer phosphoric acid. It is a mineral acid and the acid you will read of in the book
I would say most homebrewers make 2 big mistakes on belgians. First, they are chicken to use enough sugar. Tripel should be about 20%. Secondly, pH is too high. Lower really lightens it up and makes it a more authentic (gives it the champagne-like vibe). This is probably why your wife likes this style and why I pointed you in this direction...
Brewinhard is spot on. The KISS method for now would be just brew your next with RO (I prefer distilled) and that should give you an idea. I just use tap water (very average stuff) but use the acid to hit 5.2 in the mash and also acidify sparge to same. Not really hard to use acid. All you need to get started is a 10% solution from LHBS and a baby medicine syringe to measure. Run your numbers in Bru'n water and off you go.