If all that's in your water is carbonic acid from dissolved air and a little nitric acid from fish waste, the expected pH is less than 6, so unless your fish are showing stress I wouldn't worry about it. Those species will certainly be fine with pH in the 5s. If it worries you though (and I can understand not wanting anything to be outside the range of your test kit) I would suggest a solid buffer rather than regular liquid treatments and testing. It's cheaper, far easier, and better for the fish since pH will remain relatively constant rather than up and down all the time. Also many of the commercial pH Up/Down products are a phosphate blend that will tend to encourage undesirable algae growth.
What I use is crushed oystery shell, which blends in well with most gravel or sand as long as it isn't Tahitian moon sand (that nice black stuff that's hellaciously expensive). You can get it at a farm supply type store, where it's sold as a dietary supplement for chickens. I got a 50 lb bag for $10 about two years ago, used it in all ten of my tanks, and still have 40 lbs left. A similar product (crushed coral) from an aquarium supplier might be more like $25 for 5 lbs. I don't even test pH any more, and it's never been a problem. If you don't rinse the shells there is some initial cloudiness, but I quit worrying about it myself and it also has not caused any problems.
Basically I just dumped a cup or two in each tank and it slowly dissolves calcium carbonate over a period of months, raising both pH and GH. The calcium is also somewhat beneficial to the fish, and it helps out snails and crayfish if those interest you (neither will survive in a tank with those sunfish though).
Edited by gzeiger, 31 August 2010 - 06:40 PM.