It may not be legal, but as long as you don't let them get into the wild, it won't affect the ecosystem, regardless of species. Also, I am highly critical of laws that cannot, in any reasonable way, be enforced. I have yet to hear of any law enforcement obtaining a warrant to search a house because it might have... tadpoles in it. Additionally, most police officers are busy with laws broken that actually matter, or collecting income for their jurisdiction with minor traffic violations. You could probably stroll by a group of law enforcement with the container of tadpoles in your hand and none would give it a second look. Kind of like J-walking is illegal, but how often is anyone cited? Your only concern would likely be if park rangers (or whatever they are called... I prefer "Forest Cops") caught you trying to remove an endangered species.