+1 on the monitoring PH levels after adding wood to your tank. I added a fairly large piece and my PH and alkalinity has dropped quite a bit. I never had any luck with carbon removing the tannins from the water, but I picked up some Seachem's Purigen and that has worked wonderfully.
The tannins cause your water to turn brown, but they also cause a drop in your PH levels. Depending on what kind of fish you're keep, that could be a good thing. Make sure you know what conditions your fish require. You may need the buy some buffer to maintain a neutral to higher PH. I would recommend Seachem alkaline buffer. That's what I use in my tank. I have driftwood and my PH hangs around 7.6 with the buffer. It's a little higher than I prefer but my fish seem happy. Just need to calm down the dosage is all.
If you don't mind the tannins in the water, you can just put the driftwood in the tank. It's called "blackwater" and many south american fish live happily in these conditions. A little activated carbon will remove the tannins from the water.
Now thats wierd, because it actually upped my PH. My PH has always been about 7.6 too, but after I put it in, it went to 7.2 or around that.