Dead wood is a relative term when we speak of how dry it really is inside. If it is totally dry and your water has good buffering, the type of wood is not important as concerns safety for fish. Really any wood is going to be safe. IF and it's a big IF, you watch the water and keep up with changing water as needed. A bleach water soak and drying is my choice for resetting any hazards from pollution. Since it may take several hours at least for water to fully soak into the wood, boiling is not very effective other than making one feel safe.