In my 5 gallon tank I have two Black Skirt Tetras and a Serpae Tetra. My two Black Skirt tetras have fins that are splitting and, well I don't know; their fins look terrible. I've tried medicine for fin fungus and something else for bacteria months ago with no improvement so gave up. I've also seen them nipping at each other. I'm looking to add another fish to my tank. Is it safe to add more fish to my tank? Here are some pictures. Is it nipping or fungus? How should I treat it?