Are these new additions? Did you ever see them eat?
How old is the tank? what other livestock is in there?
Are they gasping, is there any spots on their body, stringing white poop, swollen gills?
What have you been feeding?
For the tang, you may want to put some Nori on a clip or wrap it around a piece of PVC pipe with elastic band.
Try some frozen mysis or call the store you got them from to see what they were being fed.