many species of tadpoles can be very inactive from my experience they will sit at the bottom or under vegetation all day. Also what are you feeding them? I have even seen American bullfrog tadpoles play dead when they felt threatened. Tadpoles have many predators in the wild. with this said they have instincts to become less noticed by fish turtles and of course there own parents.

Jen can give you much more info on this then i can.