Most woods are safe provided they are not coniferous. Some woods that are known to be deadly in your tank are
I’ve been told that anything evergreen is deadly, but I suppose it depends in the tree. I’m from a country whose main habitat types include tropical rainforest and coastal mangrove forest, so evergreen trees dominate the landscape. I’ve been using the wood for decades and have never once found them to be a hazard to my fish, heck even my plecos graze and eat the wood with no harmful effects.