I have to dissagree with aquaristdave, tannins will not harm your fish in the least. The only problem, is that it will discolor your water, and some people don't like that. If you don't want it to disscolor your water, boil it for a while then dump the water and boil again, keep doing this until the water is relatively clear, this will also help waterlog the wood so it will sink. If it is to large to boil you can soak it in a garbage can or something and change the water in the can every other day or so, this will take considerably longer though. If it is too large to boil, then you can bake it to disinfect it before the soaking process.