most trees that bear fruit like oaks and apples are fine but don't get any wood form trees like pine, they contain sap and its poisonous to fish. after you pick the right piece, peel off all outer bark and soak for a week to get rid of tannins.
tannins arent bad for your tank but they will turn it into a tea color so soak it a long time. then to disinfect it, put aquarium salt into the bucket your dumping it in. you wouldn't want any parasites or disease to come off the wood
Growing old is inevitable. Growing up is optional :3