you can use any wood, just make sure you boil the hell out of it. Boil it for hours then pour out the tannin stained water and boil it again, do this until the water isn't turning brown anymore. If you don't your tank water will be brown, and you could introduce something unwanted into your tank. If the piece is too large, well. I guess your out of luck on boiling it. So just make sure it has been out of water for a while and there is no chance of bad stuff being on or in it.