If you don't know what kind of tadpoles they are, do not offer them to your fish; some tadpoles (especially toads) are quite toxic. It also may be illegal to collect them- be sure to check local laws and regulations. And of course, never collect live food from an area that may be contaminated with pesticides or other toxins.
That said, tadpoles can be an excellent supplemental food; they are rich in fat and protein and have no indigestible scales or bones. I often feed tadpoles to my snakes and turtles.
Tadpoles of bullfrogs and relatives (ranids) and most treefrogs (hylids) are non-toxic. To my knowledge, tadpoles do not carry any parasites that are transferrable to fishes; they are certainly less of a risk in that regard than feeder fishes. If you can get some good photos of the tadpoles and what area they were found in, I or someone else should be able to tell you what kind of tadpoles they are and whether they are safe.