From The Aquarium Wiki
What is it?
Brushwood is a term for twigs and other dry dead wood that can be used in the aquarium, but make sure it is fully dry and not rotting. Brushwood contains fungal spores and is prone to rotting when it is wet. If it is placed in the aquarium without treatment it will rot, and produce slimy bacterial blooms. Boil it for an hour or more before butting it in your aquarium. Do not use pine or softwood brushwood, as the sap is harmful to fish.
Do not use pine or softwood brushwood, as the sap is harmful to fish. All of the aromatic woods contain toxic, volatile oils known as Phenols. The best woods for the aquariums are from fruit bearing trees (apple, pear, cherry, walnut, oak etc), anything 'sappy' (ie pine, cedar, beech, maple, sycamore) have high contents of Phenols and are toxic to most life forms in some respect (it has been proven that pine/cedar shavings cause cancer and liver damage in rodents). That is why pine is used in furniture and cedar is used in mothballs.