That is EXACTLY what happened to me!
I've raised tads for a long time, and when I stumbled across my tadpole in a feeder fish tank, I decided to raise him. He's now a healthy little Southern Leopard Frog. (See the thread right before this one for a picture
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First off, you'll probably want to get his enclosure right- Which is pretty simple, except you must have something for him to get ontop of and out of the water. He may not be a fully aquatic frog, and if his lungs develop before you have anything for him to climb onto, he may drown. Make it a sloping area out of rocks. That way, if he isn't fully aquatic but he also isn't a climber (tree frog), he'll still be able to kind of come up onto a beach.
Food isn't too much of a problem at this point. Once their front legs begin to come through, they stop eating and absorb their tail into their bloodstream- thus providing any and all nutrients. So if you're putting food in there and he stops eating, this is why, don't be alarmed. Tropical Fish Flakes will work fine, but I usually feed mine those along with bloodworm larvae and algae flakes.
You'll want to search google for a Tadpole Identification Guide. I have a brilliant one bookmarked on my home computer (I'm at school right now) so when I get home I will post it for you. You'll need to do a bit of research along with it but it's pretty worth it.
Without that website, that's all I can provide for now. Once you find out his species, you'll know what kind of enclosure to have.
I'm going to go see if I can find that identification guide for you. 