uh soo..i doubt many types of wood are poisons to fish
that being said almost all of my wood is collected by me from rivers and even the bay in my area.
what i do to make it tank safe:
1 .when selecting wild wood press your thumb nail in to it ..if you can make an indent it is to far rotten and will fall apart in your tank..toss it and keep looking till you fine some good solid wood.
2. let it dry...all the way give it a few weeks in the sun to bake out any nastys there might be from the water..its a good time to give it a good scraping to remove any bark or mud.
3. soak it!...as of of water as you possibly can..water changes daily with scrubbing between each change..keep cleaning till all the bits are off...give it a good 4-6 days of this treatment.
4....at this point it should be pretty good but will float keep soaking with stones on top of it till it sinks and you have your self some nice cheap drift wood!!
if the peaces are small enough you can just boil them..and skip allot of this.
or if you have a power washer you can use that as well.
a good wire brush will take almost anything off strong healthy wood with out to much damage or a wire pot scrubbie (not SOS pads)
good luck ^_^