Okie dokie, we may need to get to the root of the problem here to be able to fix this. How long has the tank been set up? What type of filter are you using? Have you been testing the water? Is there any reason that you're adding salt?
The Mixed Fruit Tetras you have are indeed dyed White Skirt Tetras - a color morph of the Black Widow Tetra, Gymnocorymbus ternetzi. They're actually a rather peaceful species, but are best kept in groups of at least six or they will fin nip. Because of this, it's highly suggested that they need a larger tank to allow for a larger group. This article on the Black Skirt Tetra is from Betta Fish's sister/parent site, Tropical Fish Keeping, and is written by extremely experienced aquarists - http://www.tropicalfishkeeping.com/profiles/black-widow-tetra/