Driftwood will not sink on it's own until it is fully waterlogged. It can take weeks or months, depending on the density of the wood. It's better if you can presoak it for a while before you use it. I always have some soaking in large trashcans in case I need a piece. I've found that some natural driftwoods get a little nasty in the tank after a while, and a large piece can be a bit of a hassle to remove and clean. The newer "fake" wood pieces made of poly resin look pretty good in larger sizes, and seem to stay much cleaner. If you keep plecos it wouldn't work I guess, but I have been experimenting with them, and like them better so far. Just something to think about.