I know of no sealers. But my suggestion would be put them in big rubbermaid containers in the bathtub and put boiling water on them, changing the water daily
Change the water your soaking it in daily.you will see that the water gets lighter in color.when you do add the wood to the tank, you can put carbon in your filter to remove any tannins left
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Any sealer you use will eventually flake off in the tank. A good marine epoxy will last a long time, but isn't necessarily fish-safe and will be shiny.