any suggestions?

[basskid081990]

i started up my new ten gallon about a month ago but since have been plagued by diseases that i dont know how to identify. i first had guppies, mollies, and tiger barbs which lived fine for about 4 days until i noticed my guppies tails starting to disapear, and as i knew all he other fish werent nipping due to hours of observation i started treating for fin rot. its then that i noticed the mouths on 3 of my fish were constantly open and they wouldnt eat. this i am unfamiliar with. with a treatment of melafix and pima fix for 7 days with a 50 percent water change it disappeared but keeps reoccuring. i have no idea how to get rid of this problem. ive had the water tested and everything is fine so im worried its a pathogen. any suggestions would be great