Mopani tends to be a BIG tannin leacher, as in they can release tannins for up to 6 months (according to one review I read about the various driftwoods). Tannins are good for the fish, but if you're concerned about your "shabby" tank having yellowy water... lol. Boiling it for a few days, or soaking it for a week or two (changing out the water every day) helps accelerate the process of leaching, so by the time you put the wood in the tank, most of the tannins are gone. I boiled my one piece of mopani for 24 hours on the stove, then soaked it in a bucket for a few more days, and it was still leaching by the time I put it in the tank (I didn't care about leaching anymore, I just wanted to enjoy the darn piece of wood, lol)