Uh, dorsal fins? I have never in my experience with caudates heard of a tadpole having a dorsal fin. But to feed it you can give it green water, like for tiny tiny fish fry, or you can give it algae drops or something with algae on it. I use lettuce for my frog/toad tadpoles but newts don't seem to be to interested in it.
Also newly hatched brine shrimp or other small water animals like that are good, and if you live in a warm area some local pond water might have what it needs if it's very young.
Hope I helped,
Betta splendens
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