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.