I would boil that wood, it will make the tannins leech faster, not to mention sterilize it. Boil it in a pot, when the water turns brown change the water, boil again, use hot water from the tap so it boils faster to speed the process.
Repeat the boil/change water/boil process several times (like five or six.) until the water is much lighter than when you started. After its added to the tank any tannins that leech further will be removed through water changes or the use of carbon.
Boiling accomplishes in a few hours, what a week of soaking would do.