I have cedar in my tank. I would just try to use really old pieces and make sure you soak them for along time to see if any tanin (or whatever that stuff is) is released into the water. If it does, I would not use that piece. I have had a piece of cedar in my old tank for several months and it did well. Now I have two pieces in my new 55 gallon. From what I understand, It is risky and can cause a major problem if you are not careful.