Another way to soak out some of the tannins is by putting the wood in the toilet tank—the tank, not the toilet bowl!—for a week or two. Flushing the toilet a few times a day, which you’re probably going to do anyway, automatically keeps the water changed.
While some people have an “eeeww, yuck!” reaction to anything associated with toilets, remember that no water from the bowl ever gets into the tank. Before I soak wood in the back, though I pour a little bleach* in the tank and let it sit for ~30 minutes, just to eliminate the kind of bacteria and slime diatoms that are present in any water container where water sits for most of the time. I also use a gravel vacuum and remove the precipitated flakes of carbonates that are on the bottom of the toilet tank, because the tap water here has a lot of added lime.
I haven’t tried boiling the wood, but it sounds like it might reduce the white slime it gets in the aquarium. The slime may not hurt the fish or invert, but it throws off the aesthetics.
* Before adding bleach, make sure there are no other chemicals in the water, like the types you hang in the tank that are supposed to keep cleaning the toilet. Be sure, also, never to put bleach in the tank at the same time you have a toilet bowl cleaner in the bowl. Always make sure you flush a toilet at least 4-5 times between putting bleach and any other cleaner, like toilet bowl cleaner or cleaners like Fantastik, Scrubbing Bubbles, Lysol sprays. Mixing many household chemicals can cause release of gases that are very poisonous. I’m sure you know this already, but I can’t stress it enough.