PH Drop

[Aspen35]

Yesterday I tested the Ph in a fairly new, but cycled tank. It was 8.2. Today, it is about 7.4-7.8. Why did it drop, and should I do a water change?

 

[Byron]

More info is needed, but this is easy to explain when I/we have that. Are there fish in the tank, and for how long (I gather it is relatively new but cycled). What is the GH, KH and pH of the source (tap presumably) water?