Boil it if you can. Otherwise, let it soak in a bucket for a couple weeks. Most wood leaks some tannins, which end up discoloring the water to a light to dark tan. This will let most of the tannins escape. Change the water when it turns dark, and soak more until the water color doesn't change. It wont hurt the fish, just usually isn't the prettiest to look at. If you cant boil it, strub it with a fairly stiff brush (like one to clean the tub) then put it in the dishwasher for 1-2 cycles. The hot water and heat dry should certainly kill anything growing on it, as well as pull out the tannins.