Also, some wood decay and rot in water, or if in water with too high or lower water parameters cause deterioration.... So you cant just use any piece of wood, it has to be certain kinds of wood... not all wood belongs underwater.....
if your have a local fish from the area the wood is from, they can have built up immunitys to the different bacterias and stuff.... but if you have fish from africa, where that wood is not found you could be introducing a new problem to your fish....
And lastly, before I really researched this, i did try it as well... the wood I used just kept floating, it made me crazy! I had to prop rocks and stuff to keep it down, and when my fish moved the rocks, up came the log floating to the top.... I always buy my wood from now on.... bog log looks the best, and Ive never had problems with it..... well, make sure to soak it overnight or your tank water will turn red.