There is always a risk to using wood (or stone) that is collected from a park or a garden
Animals...wild and domesticated....could well have pee'd on it, poo'd on it, scent marked it, chewed it etc
Not to mention those awesome little wood louse and other tiny critters who set up home in it too
Just something to bear in mind before drying it and plunging it into an aquarium.....it may have been "used" before you found it.