You can still use pines, junipers and many other softwoods, and probably many will disagree with me on that point; it all comes down to the age of the wood. Everything in it's entirety has to be dead, essentially at a cellular level throughout the piece, none can be left in tact.
Over the many years in the hobby, though a while it has been since doing; I have used almost anything and had it work for years in tanks without any negative ramifications.
Key things I always make sure of being, the wood is consistently dead, no fresh or possible living structure deep into it's core. I would then submerge in a container large enough for it, usually a 55gal plastic drum; and leave it for the course of a winter for it to completely freeze, so that the ice may do the work of breaking the remaining structure of any cells left intact.
Once spring rolls around I will give it a thorough scrubbing and once over with a wire brush and leave it in an area that will remain dry majority of the time, catch a bit of breeze or wind and in a very sunny area; to allow the wood to cure. A black or dark backdrop will increase the efficiency, but I leave it for 3-6months or more anyways.
Once baking is done, I submerge once more for a few weeks, using a bit of bleach or salt to make sure there are no hitchhikers deep within, then pressure wash entirely to remove all loose or rotting portions. I don't recommend it to anyone unless they are willing to make mistakes and learn from them, and though it takes a year or more for me to process a piece; it has worked for me for many years..
Although I still don't like to use most pines, spruce, and junipers.. I do occaisionally use some cedars. There are several woods that have many claims against them being toxic, but have not happened across any as of yet, and always test the ones I don't recognize with pond dwellers first, like seed shrimp, daphnia, snails, various beetles and nymphs before checking with some fish species.. but I also have several ponds at my disposal for such experimentation. All in all, hardwoods are far better, but I love the roots of many tree species and until petrified, are all quite soft woods to use but has not hindered me as of yet.
Edit: I should add that you should avoid using pieces found in skeptical areas, downstream of mining area's, near major shale deposits, near any industrial plants of any sort (just to be safe), and if you have access to a large kiln or oven, or have the ability to build a solar kiln which is not entirely complicated; even fresh pieces can be processed in a few days to a week if you can achieve about 900-2000*f, a few weeks to a month if less. Maple cured at 1200*f for two weeks turns a beautiful walnut colour and is as pourous as the fine honeycomb of a cars catalytic converter, an amazing amount of surface area for bacteria and biofilm to accumulate, but obviously not as simple a method to achieve and well.. a hardwood.
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