Is the red line where the normally whitish line coloration would be? Running the length of the fish starting at the tail and coming to about the end of where its guts are visible from the outside? If so, check the fish for small (about half the size of a pinhead), black spots around the back of the gills, or anywhere else on his body. If you see any, then the tang has black Ich. I had a Yellow tang develop this and die. He showed similar symptoms to what you describe.
Oriongirl says that she has treated this by lowering the specific gravity in the tank to around 1.020, and with FW baths. I have found that No-Ich Marine works very well, and it's reef safe.
Of course, your tang could have something completely different. Is it showing any other symptoms like flashing, labored breathing, or holding his fins close in to his body? Also, are you providing the tang with enough vitamin C?