I've never heard of sealing wood for an aquarium.... Oak is fine in aquariums. You need to remove all the bark and the cambium layers, so that you don't have them polluting the aquarium when they rot (they contain nutrients, where the inner wood is just cellulose). Soak it in a bucket for a few days after removal, to help it leach out the tannins (harmless, but color your water brown). It might take a while for it to sink (days to years depending on the wood) Boiling it will help it sink faster and will sanitize it. Driftwood will grow fungus on it (it's white) the first couple of weeks in the aquarium, it is harmless and usually goes away on its own (plecos and snails LOVE to eat it too!). Hope this helps and good luck.