What should you use to stabilize your aquarium?

The less you mess with your water the better things will be for you and your fish. A varying pH value will make for sick fish very quickly. Most reticulated (tap)water is OK for the vast majority of fish species. All you should need to do is make sure that when you first set up the aquarium and when you do large water changes, any Chlorine is either chemically neutralised by adding a "conditioner" or allowed to evaporate off. If you are having pH variations in an already established tank, you could use a pH stabiliser block/buffer. These are fairly cheap and should keep the water around pH7. I believe there are also various different blocks available that will actually adjust the water to suit both Alkaline and Acid loving fish too.