Osage Orange, aka Hedgeapple tree. Used long ago as fence for livestock also used to make bows. Its one of the hardest woods there is. I've had a large stump in my 55gal for 2 months now, hasn't effected ph and my fish are healthy. I would say it is fine for your tank, but maybe someone knows better.
Squirrels love the seeds, so I don't know how toxic it is really. I've heard if you put the fruit of this tree (hedgeapples) around your house you wont have bugs but i think it is a old wives tale.