Step one is to know your water. Does it have a lot of buffering from high GH/KH? If yes, the question is much easier. The wood will tend to reduce the buffers as it leachs anything into the water. This can cause your PH to drop, once the buffer is used. Lots of buffer, less trouble.
The reason for shying away from soft woods like cedar are that they do tend to have more sap ( and tannins) which have more effect on the water. Any wood that is famous for not rotting is apt to be the same. Cedar,fir, pine are all "sappy". Your counrty is apt to have some but I don't know the names. But then that is not a critical point if the wood is chosen carefully.
Lakes around your area that rise and fall with the weather? Excellent places to hunt wood! A lake which filled and covered timber 25-30 years ago will often be a great place as the wood will have been dead long enough to have leached all the sap and tannins. My theory is that the water rising kills much of the fungus and bugs which make wood rot. Then the drying does the same for the things which make wood decay when under water. You wind up with dead, totally dry wood but which has still not rotted or been eaten by bugs.
Find a likely wood that is not down on the ground for long. When picking it up it should be lighter than normal wood of the same size because it is dry. If it has no bark, and feels right, try cutting into it with a saw or break it if small enough. There should be a pretty uniform color all the way through if it is totally dry. Some different color around knots is okay. The more uniform the color from side to middle, the better. Avoid tannins if you can. Much easier than trying to get rid of them!
My big cedar piece which worked for me.
Bottom where I cut it out for space
Don't worry about type of wood as much as CONDITION. Get dry but solid wood.
My big cedar "GOTH" tank.