Sick Molly and Guppy

My 60 gal. indoor aquarium has always been clear and the water has
checked good.  4 day's ago one of my male Black Molly's started
swimming with his head up and fins moving very fast.  Just noticed
that a male guppy has exactly the same symptoms.  All the fish have
been in this tank for 3 weeks.  This is an established tank with other
healthy fish that are quite old.  Any help will certainly be
appreciated.