Chasing a sick fish is only going to stress it more. If it's still eating well, you can leave it, watch it, and hope for the best. If you decide to remove it, you need to limit his avenues of escape and avoid chasing or injuring him. Since it's a Hippo Tang, means you've probably got a fairly large tank. You might try using a piece of clear plexiglass and see if you can coral him behind the partition on one end of the tank. You might even get someone to help you by using a second net. Of course he'll run like the dickens as soon as a net breaks the water. I had to remove a perc clown once, but they sleep like the dead and I simply got him at night after the lights went out. The horse is out of the barn now, but if you decide to pull him, hopefully you've got an adequate size QT. Match pH, SG, and temp in the QT with your DT. I had good success treating ich with hypo in a QT. It just needs to be done gradually Needless to say, don't dump meds in your display. Good luck.
