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The first general guideline for wood is to boil it for several hours or soak it for a period of time. Boiling tends to kill any bacteria present in the wood and makes the wood water-logged so it will sink. If the piece of wood is too large, many people use a cheap trash can or tub to soak the wood in hot water. Keep adding hot water and scrubbing the wood for several days to a week. Soaking the wood also helps to remove tannins, but this can sometimes take up to a month to remove them thoroughly. Tannins will stain your water, but the amount of tannins leached into your aquarium will depend upon your specific piece and type of wood. You will need to replace the water in the tub every so often and it usually takes about a month for the tannins to reduce to the point where they will not impact your water color. Also, do not bleach the wood as it will be hard to remove from all the crevices and boiling does just as good of a job.