Sometimes terracotta will bubble when you put it in a tank, but it's nothing to worry about, just air bubbles escaping from the pot. My sororities loved their clay pots.
That was one of the reasons I thought I'd soak them a bit first - but I'll use the aquarium cement first. Figure I'll put it on like a "bobbin", a bit extra over the bottom outside and inside, so even if it comes off, the plug will still be in.
Uhhh - do bettas nibble on loose things like that? I haven't seen them nibbling on any of the plants, so I'm guessing not - but my cats all would!
dg