Hello. The last time I put Mopani wood into one of my tanks, I kept it in a bucket of water for several days to leach out more of the tannic acid. The water got pretty dark in the bucket and I didn't have nearly as much leach out into my tank. A few water changes later, the tank water was clear. I wouldn't fret over anything bad being attached to the piece. The chances of a pathogen living on a dry piece of wood is extremely remote. Plus, if you keep a very clean tank, you have healthy fish. Healthy fish are essentially immune to infections. i always add a bit of standard aquarium salt to my replacement water. Most pathogens don't do very well in water that has a trace of salt in it.
The stuff sold in the pet trade is the root wood from the mopani tree, which grows in the deserts of southern Africa. It's also called the turpentine tree for its sap. It's special because it's very dense, which means it often sinks immediately, or very quickly. It's more well-known among herpetofauna keepers because it's also very rot-resistant, so it can withstand the high humidity of tropical vivariums.
Commonly sold 'drift' wood in usa mostly by zoomed though there are other wholesales. It is actually pretty decent wood though a bit on the hardside for wood eating pleco but i use it to make nice caves for my loaches.
