Cedar is OK if it is weathered. Pines, firs, and spruces have toxic sap; avoid these unless they are very well-weathered. Look for wood that is gray and skeletal looking, and rock-hard all the way through; it should be fine no matter what kind of tree it is from. At that stage there's practically nothing left but lignin.
Most other trees (including practically all hardwoods) are OK but the wood still must be at least somewhat weathered; fresh-cut or green wood should not go in the tank, as it will foul the water. Also avoid wood that is decaying; if there are just a few rotten spots on an otherwise sound piece, use a drill or chisel to remove them.