How do I care for tadpoles?

I had a bowl of stagnant water outside, and noticed tadpoles lately. So far I've fed them lettuce as recommended by various websites, but I notice that changing the water methods vary. Their water has never been changed. Should I change a part or all of it?

Also they're outside and its about 54 degrees farenheit lately. I read that they need about 60+ degree environment. Should I just keep them outside? It could go below freezing here, but probably not much colder than that.

Thanks!