The waste your fish produce and any decaying matter in the tank will cause the PH to drop over time, how do you do water changes? do you scoop off the top or use a gravel cleaner?
A difference this great between the two sounds suspicious, do you have a lot of bogwood in the tank? It should be pretty similar if you are doing regular water changes and everything else is equal. I suggest you test the hardness of the water either with your own test kit, or a fish shop might do it for you. Post back with the results as soft water PH fluctuates more easily.
Ken