A few comments on wood in response to several previous posts in this thread.
There are several different types of wood available as aquarium bogwood. Some are bad for tannins and some are not. Generally speaking, the lighter woods (like mopani I think it's called) will leech tannins for quite some time. The darker wood (ironwood, mangrove root) lose their tannins fairly quickly, sometimes just a boil or soaking for a couple weeks. I always use the dark woods because (a) the tannins are gone fast, (2) they are more natural with plants in my view, (3) they tend to last underwater--some woods rot sooner than others and this can be troublesome. In my experience the darker woods stay firm for years. [I didn't check the link to ebay, so don't know what was being offered.]
Second, I always now buy wood that is in a tank of water, usually this is with fish. I can tell from the tank that the wood has leeched most of its tannins. All the wood you see in my current setups came this way, and I have never once had coloured water in the aquaria from it. If it comes from a tank with fish, soaking it in a pail of plain tapwater for a week will handle any parasites like ich.
Tannins will not hurt fish or plants. Some like the yellowish-brown colouration it gives to the tank, others don't. But there is no harm from it. It will lower the pH by acidifying the water, but even with lots of wood this is minimal, perhaps .2 at most, so wood is not a practical means of lowering pH nor will it cause problems by doing so.