There is a good bit of difference in tadpoles no? I understand the little black ones are toad tadpoles which fish wont eat at all, but a bullfrog tadpole I would have guessed they would eat, maybe they have some kind of deterrent too till they get a little further along, natures way of letting them get a start, with out that they would have no defense mechanism to keep them from pretty much 100% eliminated in a short time in any average pond.
