In my young boy days I raised quite a few tadpoles to frogs. All I did was take a bucket with me, fill it with pond water/algae then started catching tadpoles. I left them in that, once their legs started growing I tossed in a stick for them to crawl up on and eventually get out on their own.
We've just recently started getting into fish and found out that you don't want to do a 100% change out of water if you don't do it very frequently. They get used to their own "soup" and suddenly adjusting to clean water can shock them to death. Makes me think tadpoles have the same issue.