It doesn't get fungussed. Tannins are not toxic and some people like the colour their water takes on. If you don't, or if you want to keep things under control, use carbon in your filter. If you do not soak it for a long time, then you may find that the wood will grow a white fungus layer on it. Repeated soakings in hot water seems to prevent whatever it is that makes the fungus grow.
Mopani wood looks nice once it has stabilized.
But it can take months before it stops leaching significant amounts of tannins. At the lfs they told me to soak it for three days. I think they just want to sell as much of the stuff as possible and if they tell customers the truth about how long it really takes until it is O.K. to leave in the tank, then nobody would buy it. I would recommend doing it with any piece of wood, both for sterility reasons and for bouyancy reasons. If you boil it, you kill virtually every pathogen known.