There is a Wiki article on the medicinal use of Witch hazel, and it lists the ingredients that are in commercial witch hazel, when you buy it as an astringent for external use. While some things are fine in a tank, (comes fromo peat moss or many species of driftwood) I do not see how they would help you clean the equipment. Some of the other ingredients I would not want to have my fish exposed to without knowing more about them.
Also, if some of the ingredients in witch hazel are naturally found in wood we more commonly use in aquariums, or in peat moss, these ingredients are safe in the aquarium at the levels released by the driftwood or peat. Perhaps the more concentrated ingredients in the medicinal form of witch hazel are too strong.