I'm not sure why the Guppies don't survive. I had 5 in total, bought from 2 different shops and now I only have one left. All of my water parameters are spot on. They just spend their time at the top of the tank and won't survive more than a week. I have a book which talks about the fact that they used to be a hardy fish but now with the amount of inbreeding, they have become quite fragile. I won't bother with anymore Guppies, I really don't feel good about them dying off.
My driftwood was tagged as 'Mongolian Gold Vine' I believe. It wasn't cheap, but I think it was worth it. It did not leach out any tanons and it didn't take long to make it sink either.