You certainly can't just use any wood in a tank, no. Some produce resins that foul, or even poison, the water.
The very safest way is to buy your wood from a shop. You can collect wood for yourself, but you must be able to identify the tree it came from and it needs to be long dead. You need to avoid all soft woods (like pine) and most nut producing trees (hickory, walnut)
I don't know what trees you have near you in SA, but you're safe with fruit tree wood (apple, plum, pear).