Regular water changes are a necessary part of your tank routine in any case, so unless you have been doing that I would start. Having said that you do not really want to change the pH by more than 0.3 per day as the pH scale is exponential - meaning that for a 1.0 increase in pH it changes by a factor of 10 (pH 8.0 is ten times more alkali than pH7.0) So, you may find you need to do several water changes over the next week and then maintain a more regular water change regime in the future.
Over time, if the water is not buffered in terms of hardness and alkalinity, stuff like fish waste, the carbon dioxide they expire, nightly CO2 release from live plants, tannic acid from certain types of wood etc will all increase the acidity of the aquarium and cause such a pH drop.
Check what it is out of the tap too though (although its unlikely to be pH6.0)
Hope this helps.