I've got several bits of "garden tree" in my tank. A dead-branch of the cherry tree for instance works just fine. Oak,
beech,
apple, orange, peach, etc, etc. I'd stay away from pine, spruce, cedar etc.
A piece of for instance mahogny would surely work nicely too - any small piece from your local DIY-shop or builders merchant.
Any dried wood will need a thorough soaking to sink - for bigger pieces, it may take several weeks. It's fine to just quickly clean it (boiling or dishwashing machine if you want to make sure it's insect/pest free), soak it for a few days in a tub of some kind, then rinse it off with clean water, and weigh it down with some rocks or soemthing in the tank to soak it further.
There was a discussion a while back about different types of wood - if you search the forum, you'll probably find it.
By the way, both Panaque and Ancistrus species are able to gnaw on Mopani, which is hard, heavy wood, so I'm not sure it has to be that soft...
Obviously, if someone knows of some "aquarium ready" soft-wood, I'd like to hear about it.
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Mats