· Premium Member
Tannins are not harmful to the aquarium if they are released slowly. They discolor the water, which you might not like, but as Robert said, you can use carbon in the filter to remove it, or just do more frequent water changes. When I had a big piece of wood I wanted to use in my tank I used a big plastic ice chest to soak it with hot water - the ice chest was the only container I had that would hold it. After a few changes of water over a month's time I declared the wood to be ready for use in the tank. It still discolored the tank water, but water changes kept it from being a problem.