Very buoyant mollies

I've never heard of internal fungus', it's probably either overbred
white mollies (I seem to remember having read that mollies are
naturally black) as stated by sophie, or a bacterial infection, ask
around and maybe try a full dose (so the bacteria doesn't get the
chance to evolve and adapt to the anti-bacteria medicine and become
resistant) of an antibacterial medicine. Remember to research the
medicine before you use it as you might very well ruin/damage a
significant number of lifeforems (including good bacteria) in the
medicated tank and try to use a hospital tank after researching the
pros and cons of such too. Good luck!