More details are posted in a thread 'What are KH and GH?' in Freshwater Informations sticky thread.:)
pH adjusting chemicals should be avoided. They'll do more harm than help. Once the effects wore off, the pH swings back and if the difference is quite big, the fish will become stressed and eventually killed.
Tannic acids produced by peat and driftwoods, and CO2 both can lower the pH. Depending on your hardness levels, the pH will continue to drop if the KH is not high. KH must be maintained above 4.5 dKH. Anything lower will cause dramatic pH swings which will harm your fish.
Your fish are doomed if pH swings do happen or pH lowering further. 5 is really too acidic for most fish and should be stopped from decreasing further.