You may want to clarify this statement from the other thread. If you are topping up with salt water, then everything in your tank will eventually die (but I hope that isn't what you meant).
That narrows it down to about 20 diff ones, some of the radianthus get huge. Could this fish have been stung? Pretty active swimmer in a small tank might bump into the anemone.
The salt doesn't evaporate out of your tank, so as water evaporates the concentration of salt in the water gets higher. Since the water that evaporates out is fresh water, you need to replace it with fresh water to keep the salinity at the same level. If you add saltwater you are increasing the concentration of salt and it will continue to get higher and higher.
I don't know that it has been stung for sure. I have never seen odd marks like caused by any disease. I have had fish stung by carpet anemone that made it. these were wrasses though a little bit tougher than that tang.
PitPat is right about the salt, you need to check the salinity right now.
My tang used to go like that in the evening, or when some thing anoyed him. It never harmed him. He grew and grew till I had to find him a bigger home.I never saw him near the quadricolor anemone so I dont think it was a sting.
Srry to here that. Make sure your salinity is 1.025, not .025. Makes a huge difference.
Don't add any more fish to the tank for a while until you get this figgered out. I use to have a huge ich outbreak and sometimes patience is the best remedy.
However, something is causing this in your tank, and I would find it out before you add anymore fish.