Tadpole with air bubbles under his skin?

I recently put three small tadpoles in a 20 gal aquarium that I am keeping outside. They are just regular pond frogs/toads. They are growing well and eating vigorously, but then the smallest one starting spending a lot of time near the top of tank. Then I noticed he was upside down and has two air bubbles that I can visibly see under his skin. He can't swim to the bottom of the aquarium because he floats immediately back up to the top. He tries and tries to get to the bottom but the bubbles pull him back up. I'm am so puzzled! Does anyone know what causes this or how I can cure it? I made a full water change and I'm hoping that will help but I don't know what else to do. Also, how long will it take to go away? Will he survive? Thanks!