Actually sound advice from your LFS.
Trying to alter the pH with buffers is just that--a rollercoaster ride.....
If you want to understand the reality of pH in the aquarium and the myth that the pH crash is--take a read through these threads:
Low or no KH and low PH without a "crash"??
Low KH and pH crash
pH = 5.5, what now?
Theory is Great, but reality is what matters......
RO isn't expensive really to run over the long haul, but it can be a hassle. It can be pretty expensive to set up (~$300 maybe more, maybe less--depends). You said that your dad has a carwash--if it offers the spot-free rinse--then you have all the RO or RODI that you'll ever need. You'll just have to transport it. Agian, to me--that would be a hassle-factor.....