If this is driftwood your buying from a pet store. It could slightly lower your ph and leach some tannins into the water darkening it slightly. This is actually good for certan types of fish neons, cardinals, etc. If however this is something you collected it could have all sorts of effects (some could be terrible) if not properly cured. For example beach driftwood can leach harmful amounts of salt, others could have absorbed contaminants which in the confines of a tank can be lethal.