I have to agree more with fifefish, however what ak>47 said is true as well.
Like stated above, wood that is "meant" for aquariums are usually roots and what not, wood that has been in water already.
If you pick wood up off the ground, that has not been submerged in water yet, you are going to risk the wood rotting over time in your tank, and fungi and what not as well.
What should I do if I find a piece of wood that I want to put in my tank.
First, shake, brush, whatever you have to do to get any loos wood off of the wood.
Second, boil the wood for sometime. When I bought a piece of wood for my tank, I boiled it for 3 days, and soaked it in water for 2 weeks. Boiling the wood is going to make the piece of wood become water logged faster, which will allow it to sink to the bottom, and it will leak out tannins faster.
What are tannins exactly?
Tannins, are a natural extractive from wood, which will leak out of the pores in the wood when the wood becomes submerged in water. The color of tannins is usually a brown/yellow/gold color, like a tea color. Tannins are NOT harmful to fish by any means, and some fish will actually like the tannins in the tank.
After I have soaked the wood for a couple of weeks, and boiled the wood, and placed it my tank, why is the water still turning brown?
Even after soaking the wood, and boiling a lot, there are still tannins that wili most likely going to leak out into the tank for sometime, although it may not be that much.
The only way to get tannins out of the water effectively it by water changes, and by carbon, or something similar to carbon added to the filter.
After an "X" amount of time, the tannins will stop leaking out, and your tank will be clear.
-FHM