That tank is a nice find.

Tough choice with such a big tank and you might find your going to want a nice size group of them. Leucs are definitely the easiest.
Since everyone has blue frogs, I would be hopping on the Mantella bandwagon.
Mantella milotympanum | Understory Enterprises

I don't think they would be the boldest like Leucs but should be a ton of fun to watch in a group.