Anything that will leach out whatever is in it.... I wouldn't use eucalyptus, I don't think eucalyptus oil would be good for fish. (you can't give it to birds, except for native birds, it can be toxic, it's toxic to people too). That's the difficulty with finding driftwood - knowing exactly what it is to be able to look it up as to whether it is safe and then there's the risk of sprays or other contaminants. Even though shop driftwood is expensive, it saves that worry. Unless you can find something secondhand, I think you just need to bite the bullet and hit the shops.