You might want to try boiling it for much longer than that. I boiled two small pieces (the larger of which is about as big around as my thumb and a foot long) for close to 10 hours and let them soak overnight before they would even begin to stay down in the tank. Granted, they were close to completely dried out when I started, but 30 minutes isn't going to cut it.
You only need to either bake it or boil it in water to sterilize the wood, not both. Boiling the wood is the prefered method, since it helps release tannins and helps make the wood sink easier. Baking would dry out the wood too much and you may end up burning it if you leave it in too long.