dying tiger barbs

Hello, to start of I will be as specific as I can. I have a 20 gallon aquarium, newly cycled, about 4wks when I started with the fish. I had been testing the water everyday. All was good. I bought 5 barbs. (Only was going to do 2-3 but also knew they are schooling fish, and he said 5 should be...

As the title suggests I have a community tank but only my Tiger Barbs are dying. I woke up this morning and everything seemed fine, an hour later 2 were dead, 1 died this afternoon, and 1 died this evening. Does anybody have any idea what could affect my tiger barbs only? So you have a bit of...