I would gladly use that root in my aquarium! I would wash it well with pressurized water, possibly with a pressure washer. Then put it in a big container and pour boiling hot water over it until it is submerged, and let it sit for a few days. Then I would consider it safe for the tank. If necessary I would attach some pieces of granite or slate to the bottom to hold it down in the tank, but root wood tends to be denser than limb wood, so it might sink anyway.
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