Well, there's three questions one has to ask: Why do you want to mess with your PH, what is it now, and where do you want it to end up?
If you're only wanting to change it because it's a few points away from what a book says the fish "likes" then you're pretty much wasting your time. PH, except in -extreme- situations, isn't -that- important to fish...unless you have some super-sensitive species, or are trying to breed a PH sensitive species.
Depending on where you are and where you want to go, I've read about adding peat, crushed corals, and driftwood into the tank/filter to help adjust PH to certain levels. I'll leave it up to someone who knows more to explain which goes which way, and the benefits and detriments to each.
