Tetra bleeding internally???

Black Skirts may fade in color, or turn darker, but red is extremely abnormal. If these are the only fish displaying these symptoms, you're probably dealing with septicemia, which is a blood infection. Using something like Maracyn TC, Maracyn 2, Furan 2, Triple Sulfa, Trisulfa, Metronidazole, Hexamit, Flagy, Kanaplex, or Terramycin would be ideal. If using tetracycline or minocycline based products, keep the tank dark for the duration of the treatment, as these degrade with exposure to light.