Rex gave very good advice. Can't believe they actually said not only to stop doing water changes, but actually thought that the product Cycle would actually effect pH. They obviously have absolutely no idea what they are talking about. The driftwood releases tannins into the water and it is what causes the pH to lower. Did you pre-soak the driftwood before you added it to the tank? If not then it is highly suggested to set it into a bucket fully submersed in dechlorinated water (i.e. use a few drops of your water conditioner to the bucket before adding the wood) and keep an eye on the color of the water. Over time it will most likely start to darken a bit in color (kind of like tea), and this is the tannins leaching from the wood. When the water stains a bit, dump it out and replace it with more dechlorinated water. Keep doing this until the water stops changing color (this could take weeks by the way) for at least a few days. Then it's ready to put back into the tank.
Here's a site that talks about driftwood for a little more info. As for the tannins that are already in your tank, do 25% water changes for the next few days and you should eventually notice an upward swing in the pH. It sounds like you have fairly soft water for the driftwood to drastically change the pH like that as quickly as it did. To test you need a kit that measures your waters KH level as Rex mentioned... the lower the number the softer your water. Now soft water is not a bad thing per say and can be left alone but understand what it means and what you have to do to deal with it. Basically with soft water, you don't really have a buffer to stop things like pH swings. With a buffer you might have been able to add the driftwood without any pH swing because the buffer actually (if high enough) would absorb all of the tannin's pH lowering ability. If the wood released enough tannins to surpass the buffers abilities you would still get the downward swing, but it might not be as strong since the buffer absorbed most of it. With out that buffer you can still have a happy tank, but you would need to be more careful about introducing anything to your tank without knowing what the effects might be. Also a lot of live plants in a soft water tank might cause pH swings that are rather steep between it's day and night cycles. Plants release oxygen and burn CO2 during the day which raises the pH in your tank, however, at night plants actually burn oxygen and release CO2 which lowers your pH. So a little buffer to curb a bit of that action might not be such a bad thing. If you want to leave things as they are and just see how things pan out (usually a good idea... i.e. cross that bridge when you get to it type thinking) then just keep testing your pH and other qualities of your tank to see if anything gets out of the norm. Messing with the pH can be a tricky thing for even experts to handle so it's not normally recommended, but finding the KH of your tank water and your tap (to be sure they are the same) could be helpful in understanding it you need to dive into that stage of fish keeping or not.
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