Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You might soak it in water that is very high in tannins, but they would come back out, I think, when you put it in the tank.
Several kinds of oak leaves, twigs and bark are rich in tannins. If you can get some, try Quercus douglasii, the Blue Oak. Makes the water such a deep wine red you cannot see into it.
If that does not stain your wood, I do not know what might, and still be safe for fish.