Recommendations for prospective tadpole keepers:
1. Collect your own, since you'll likely be releasing them once they reach adulthood. A tadpole sold in a pet store is very likely not a native species. Amphibians are having enough trouble surviving as it is without our introducing nonnative frogs to even more places than they already have been.
2. Be sure you can provide plenty of food in the form of algae or some other tender plant material (blanched zucchini, etc).
3. If you want to keep multiple tadpoles, try to collect animals that are about the same size. Cannibalism is common practice among many species.
4. Watch development carefully. Once the tadpole has fully formed front legs, it will not be long until the tail is absorbed. You'll need to release or re-house your frog when just a tiny stub of tail remains.
5. Don't get too attached. Although many of our native frogs are small enough to keep in a terrarium, many others are not. Bullfrogs, pig frogs and leopard frogs are too big, and their long, powerful leaps will ensure snout damage when they collide with the glass. Choose a release site near where you collected the tadpoles, unless you found them in a roadside ditch.
Tadpoles are endearingly cute, and I like 'em. For really big tadpole fun (maybe too big for most people), collect a whole raft of frog eggs and raise a hundred or more tadpoles like you would a fish spawn. Release them in a nearby pond, and enjoy listening to "your" frogs singing on rainy evenings.