Once sterilized by boiling, bleach or as mentioned above (though I'm not sure of that method because bacteria/parasites might have homes in deep cracks). You should be OK. Softer wood might lower your pH, and you'll find it will continue to rot in your tank. You likely will get tannins leaching also. Assuming it is cedar, remove it every 3-6 month and scrub it with a wire brush to get the soft outer wood off. Monitor your water quality.