The bubbles you see are naturally part of the mouths. You just noticed them now. The worms you see are probably just free-living nematodes. I would not worry about them unless the activity of the tadpoles changes. If you feel like you absolutely have to get rid of them, a low concentration of methylene blue (like what you would use in a fish tank to get rid of fungus) will work very well. Use a third to half the dose recommended for fish tanks, but remove any plants prior to use.