The yellowing in the wood is called,Tannin, similar to the liquid that comes from a tea bag. Mopani wood is notorious for leaching out tannin. It is also notorious for harbouring fungal spores deep in it's cracks(if you have any). I suggest before you use it, use an aquarium disinfectant and soak the whole lump with it. Leave it for a little while and then pour boiling water over the wood to wash off the solution. Warning, be careful not to scald yourself in the process. Then add the wood to your tank and place some good quality charcoal in your filter system. The charcoal will absorb any of the tannin that may bleed out from your wood as well as absorbing any toxins. Remember not to leave the charcoal in your filter for more than 2 weeks otherwise the charcoal will dump the toxins back into your tank.
